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Thread: Total dissolved solids revisited-- tds and test-kit comments

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    Total dissolved solids revisited-- tds and test-kit comments

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    Body fluids are a mix of dissolved stuff in water that has a total dissolved solids (tds) concentration fairly close to ocean water. Coincidence? Evolution? IDK, but that's the way we all are (not just fishes).

    Osmosis is the phenomenon where fluids try to equalize the concentration of those ions (dissolved elements) across any water-permeable barrier (e.g., cell wall). That is, fresh water tries to dilute saline water if it can flow through the barrier. It's a reversible process, so you can force water out of the saline side and into the fresh side by applying a bit of excess pressure, which is how we get pure water by "reverse osmosis (RO)."

    To maintain useful amounts of the essential electrolytes for normal cell function, fresh-water fishes had to develop a complex 3-level system that adjusts cell walls to keep the outside fresh water out at just the right rate to permit normal metabolism. Sudden changes in the tds of their water can cause a disturbance to the regulatory process that can be damaging or even fatal.

    [Essential blood electrolytes include the ions of sodium, potassium, calcium and magnesium. They and trace elements like iron, copper, etc. are essential to life. Try raising fish in distilled water to see how quickly the fish sicken and die in their absence!]

    Ocean-run salmon hang in the river entrance before running upstream to spawn. It may take from 1 to 3 days for their cell metabolism regulators to adjust to the fresh water. Killies, being smaller, take from a few minutes to an hour or more to adjust their cell walls when their water is suddenly different.

    Technically, tds is defined as the solid residue left when the water is vacuum evaporated, usually expressed as mg/L. Meters actually measure conductivity of the water, which is probably even more closely related to osmosis than true classical tds. It is the ionized dissolved stuff that produces osmotic pressure, so conductivity is an excellent guide. We need meters that have been calibrated to read tds, since few of us can remember how to convert microSeimens to equivalent CaCl tds or whatever it is.

    50% or more of the myths about pH shock probably are actually due to damage from tds change and osmotic-pressure shock. The rest are due to ammonia poisoning from sudden pH shift upwards releasing toxic ammonia from ammonium in shipping water, etc. Fish don't feel pH any more than you do. You need a pH test to assure that any ammonium can't turn to ammonia at pH above about 7 or 7.5.

    Ammonium/ammonia test kits are utterly worthless. Those using Nessler's reagent cannot tell if the ammonium has been safely sequestered in "Prime" or "Amquel" and they pollute the environment if you don't have a safe way to dispose of mercury compounds. The salicylate tests are slow and still too insensitive to give proper warning. Harmful levels of ammonia are in the 5-8 parts per (US) billion, but the least step that you can see is 250 ppb! It's not like horseshoes or hand grenades (close is good enough), they aren't even close!

    My fishroom practice is to reduce osmotic shock as much as is easy to do. I try to never subject an already-stressed fish to any large change, and my upper limit on any fish is about a factor of two of sudden tds change. I'll move healthy fish from a tds of 200 ppm to either 100ppm or 400 ppm water without a lot of worry, knowing they can adjust to that much. Any more, and I set up drip acclimation to give them a very gradual change.

    [You don't need an accurate meter for such tests. Get one with the widest range, if you can. 10 ppm least count is usually better than 1 ppm.]

    I know of cases of guppies jumping from "fresh" water to sea water in an adjacent tank and surviving with little or no damage. Their initial water was probably a bit hard and salty, but the dehydration of their skin and gill cells apparently was not an irreversible effect.

    Going the other way, from hard water to soft causes the cells to swell (like your finger skin after a long bath) and perhaps even to burst. That phenomenon is a great way to either kill the fish right away, or make them susceptible to parasitic invasions like Velvet or Ich. Your first clue is often gasping at the surface as the damaged gill cells cause them to suffocate. [Fingers are way tougher than delicate gills!]

    My tds meter is my 2nd-most-frequently-used water test (thermometers are #1). I have 2 pH meters, but they hardly ever get used. Most folks need a chlorine test kit (cheap at the swimming-pool shops and spa places, but ex$pen$ive at the LFS). My well water (85 ppm tds, BTW) needs none as we have no countable bacteria.

    The advanced breeder should get a better handle on essential electrolytes by having a GH/KH set. One lets you know about the Ca/Mg in your water and the other measures the alkalinity or buffering capacity. I learned the hard way that soft water can contain too little potassium, calcium and magnesium and that adding salt to raise tds is really, really poisonous. I even killed off most of my Java Moss before I caught on. It was the same strain of Java Moss that had survived 50% sea water with my Pantanodon stuhlmanni using my hard (450 ppm) Fremont water (but not the 30 ppm Sierra snow runoff of Modesto). [That was 0 GH and (maybe) 1 KH.]

    If in doubt, always add some "No Salt" potassium chloride with the NaCl. Grocery stores usually have it for folks on low-sodium diets. You can also use reef salts which have the essentials, already.

    I used "Equilibrium" to bring both above 4 and my plants and fish immediately turned aound and thrived. I could then add salt with some impunity, as the other electrolytes were now available.

    HTH

    Wright

    PS. Will the moderator(s) please move this if they would like it in another category? Also, can a pointer be added to Farang9's request that I elaborate on this topic, in another recent subject? <A. amoenum Sakbayeme> I guess I could do that but not sure of the neatest way.
    01 760 872-3995
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    With the exceptions of the following issues, Wright's post was otherwise right on the money IMHO.

    Body fluids are a mix of dissolved stuff in water that has a total dissolved solids (tds) concentration fairly close to ocean water. Coincidence? Evolution? IDK, but that's the way we all are (not just fishes).
    Not really... marine fishes suffer from dehydration where the sea water draws water from them so their salt content is lower than the water. The reverse is true for freshwater fish.

    Meters actually measure conductivity of the water, which is probably even more closely related to osmosis than true classical tds.
    No, not really. Conductivity is a measure of the charge carrying capacity of the water. A 200 ppm NaCl solution will give the same conductance as 100 ppm of CaCl2 as calcium carries 2 charges and sodium 1. The ppm readings from conductivity meters churn out are approximates based on the theoretical proportions of the various salts. It is assumed that sodium is far more common than Ca and so wil give a reading favouring sodium concentration in a 50/50 mix of Na and Ca even though Ca supplies most of the charge. So, if you have 100 ppm of sodium and 100 ppm of Ca the TDS reading will come out 150 ppm not 200 ppm. This is irrelevant to us as we only need a relative standard to work with. I must also add that my recollection of these "facts" (if I may be so brazent) is somewhat fuzzy and no doubt Wright will correct my mistakes. :-)

    Conductivity also has about 0 to do with osmosis. Nerve cells maintain a high electical potential accross their cell membranes and it doesn't effect the motion of water which is what osmosis is about. For osmosis it is all tds and different salts have different effects on water. A 100 ppm solution of calcium will have the same charge as a 200 ppm solution of sodium... guess which one will draw more water...

    I must state that I have performed no experiments to test the statements in this post. I have instead drawn on my somewhat fuzzy experiances in biology classes where the subjects were discussed. From my recollection sodium is far more hydroscopic (draws more water) than calcium.

    tt4n

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    Hey, Tyrone. Time to erect your flame shield. You have been warned, so here goes:

    Quote Originally Posted by TyroneGenade
    With the exceptions of the following issues, Wright's post was otherwise right on the money IMHO.

    Body fluids are a mix of dissolved stuff in water that has a total dissolved solids (tds) concentration fairly close to ocean water. Coincidence? Evolution? IDK, but that's the way we all are (not just fishes).
    Not really... marine fishes suffer from dehydration where the sea water draws water from them so their salt content is lower than the water. The reverse is true for freshwater fish.
    My statement is precisely true. I said "fairly close to" and salt-water fishes aren't at all far off. Pick your nits where they are worth the picking, please.

    Meters actually measure conductivity of the water, which is probably even more closely related to osmosis than true classical tds.
    Quote Originally Posted by TyroneGenade
    No, not really. Conductivity is a measure of the charge carrying capacity of the water.
    No, that is misleading, if technically defensible. It is a measure of the ability to conduct an electrical current in the presence of an electromotive force. [BTW, know why you can't measure it with an Ohm-meter? MicroSeimens are just inverse MegOhms aren't they?]

    Quote Originally Posted by TyroneGenade
    A 200 ppm NaCl solution will give the same conductance as 100 ppm of CaCl2 as calcium carries 2 charges and sodium 1. The ppm readings from conductivity meters churn out are approximates based on the theoretical proportions of the various salts. It is assumed that sodium is far more common than Ca and so wil give a reading favouring sodium concentration in a 50/50 mix of Na and Ca even though Ca supplies most of the charge. So, if you have 100 ppm of sodium and 100 ppm of Ca the TDS reading will come out 150 ppm not 200 ppm. This is irrelevant to us as we only need a relative standard to work with. I must also add that my recollection of these "facts" (if I may be so brazent) is somewhat fuzzy and no doubt Wright will correct my mistakes. :-)
    This seems so entirely irrelevant I'm at a loss for where to start the "correction." Tds on a conductivity meter is calibrated correctly for only one substance. It traditionally is a soluble Ca salt, CaCl as I recall. This has to do with atomic (molecular) weights and only partially to do with the valence of a given ion. The conductivity "tds" will give different dry results for different solutes, based just as much on weight as on charge. Big deal.

    I said nothing whatsoever about the magnitude of the charges on different ionic species. [I did say a lot about the interdependence of all the electrolytes. That is important.]

    Quote Originally Posted by TyroneGenade
    Conductivity also has about 0 to do with osmosis. Nerve cells maintain a high electical potential accross their cell membranes and it doesn't effect the motion of water which is what osmosis is about. For osmosis it is all tds and different salts have different effects on water. A 100 ppm solution of calcium will have the same charge as a 200 ppm solution of sodium... guess which one will draw more water...
    You are misusing the term "charge" and it hasn't that much to do with understanding osmosis, anyway. The solution has a neutral charge, under most circumstances, with anions and cations in precise balance, anyway. The relevant charges are only at a sub-microscopic stage.

    Quote Originally Posted by TyroneGenade
    I must state that I have performed no experiments to test the statements in this post. I have instead drawn on my somewhat fuzzy experiances in biology classes where the subjects were discussed. From my recollection sodium is far more hydroscopic (draws more water) than calcium.

    tt4n
    The size of a molecule, and its polar properties determine whether it can be drawn or pushed through a membrane in a given condition. The polar properties of the Mickey-Mouse-head-shaped water molecule allow it to be drawn through a permeable membrane by nearby ionic charges on the other side. To the extent those charges relate to conductivity, they have *everything* to do with how osmosis works. It isn't some magical property of a salt. it is the charges and membrane properties that determine how much pressure can be built up. Period.

    We are not trying to do a quantitative analysis of how much sodium vs how much Ca is in the water. We are concerned with gill damage and preventing it.

    Again, nitpicking defeats the purpose of the posting, which is to alert folks that a grasp of osmosis and the effect of tds on osmotic pressure is quite important to good fish husbandry. I feel my posting was on the money if it convinces a few folks to get a tds meter and use it. Confounding the issue with mostly irrelevant trivia seems to defeat that purpose, to me. YMMV. :wink:

    Wanted another water-dowsing emoticon here to help Tyrone with his demolished flame shield, but the forum was too smart to let me have more ammunition.

    Wright
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    While the ruby element in Wright's laser cannon is cooling down, here is
    an URL where you can order the Hanna TDS tester:
    http://www.omnicontrols.com/search1/...er_quality.htm

    The bad news is that it is now $46.50

    You can also do a search on ebay for TDS meters--bargains have been found! :wink:

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    Quote Originally Posted by farang9
    While the ruby element in Wright's laser cannon is cooling down, here is
    an URL where you can order the Hanna TDS tester:
    http://www.omnicontrols.com/search1/...er_quality.htm

    The bad news is that it is now $46.50

    You can also do a search on ebay for TDS meters--bargains have been found! :wink:
    Bill,

    Both the tds 1, which I own 2 of (and is too sensitive) and the better primo are only $29 all over the web. Do a bigger search to find them.

    BTW, I like dogpile.com as it uses *all* the major search engines.

    BTW2, US$29 is factory-suggested price for either, so you should be able to do better.

    Wright
    01 760 872-3995
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    Thanks, Wright, dogpile.com is quite useful

    There are several vendors on ebay selling the old TDS1 for $14.95 to
    $19.95--these are nonwaterproof. The cheapest price for the "Primo" so far is $39.95: 0-1999 range with 1ppm resolution. Haven't found any meters that have 10ppm resolution and go to 10,000 in range(?), but I'm looking :wink:

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    Would the same idea apply to crustaceans? There is the common complain of shrimps like yamatos succumbing to "pH shock" upon introduction to home tanks.

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    With the risk of getting seriously wounded...
    I would like to state that my remembrance of the biological classes I took is the same as what Tyrone wrote above.
    Erik Thurfjell
    SKS 138, BKA 838-05, AKA 08998, SAA 251

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    I found a range of TDS meters under this site:
    http://www.tdsmeter.com/products.html

    All the 3 handheld models (TDS 3, 4 and 4TM) have the measurement range of 0 - 9990ppm. From 0 to 999ppm, the resolution is 1ppm and from 1000 to 9990ppm, the resolution is in increment of 10ppm.

    Cheapest (USD 29.80) one which I managed to find is Hanna TDS meter under this site but the range is only from 0 to 1999ppm.

    http://www.purewater4u.com/install/t...ructions.shtml

    However, I am not too sure of the use of this TDS meter. Do I need to measure the TDS before transferring the fish to another tank and every water change? If the difference of the TDS is greater than 200ppm, then what should I do?

    Regards,
    Ong Poh San

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    I would like to state that my remembrance of the biological classes I took is the same as what Tyrone wrote above.
    Thanks Erik.

    See Wright, I'm not totally mad!

    Because, like you, I feel TDS is very important I will not fight on and risk obscuring the important bits.

    tt4n

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    Quote Originally Posted by Pohsan
    snip...
    However, I am not too sure of the use of this TDS meter. Do I need to measure the TDS before transferring the fish to another tank and every water change? If the difference of the TDS is greater than 200ppm, then what should I do?

    Regards,
    Ong Poh San
    There is nothing magical about the 200 ppm I used as a typical example, Poh San. The important factor is to measure old and new water and make sure one is not different from the other by 2X or more. The readings are very quick, compared to a pH pen, so it is a minimal task to check as often as you feel the need. Preparing shipping water and receiving shipped fish are where I'm most careful to check tds and pH and make sure I don't add to their shipping stress. I almost never buy store fish, but bringing them home is another place where it is vital that you test carefully.

    Routine water changes can be done without measurement, as adding 25% or less of distilled water (or maybe even sea water) will not usually change the basic tds of the average tank by 2X. 95% of the benefit of most LFS medications is the instruction to change 25% of the tank water. That is almost always safe from an osmosis point of view and gives remarkable boost to the medication. Actually , you can buy the medicine, follow instructions exactly, and often get even better results if you do the water change and just don't irritate the sick fish with a questionable medication. After not applying the suggested number of doses, just throw it away and buy another bottle.

    Let's say your typical tank water reads 150 ppm. You get some fish by mail or from the store and the water in the bag reads 450 ppm. That is a difference of 3X and is likely to not kill the fish right off, but is very likely to do some gill damage and make the fish fall prey to any common parasite already in your tank water. To minimize the transfer shock, dump the fish and its water into a bucket or basin. Siphon water from the tank into the bucket via an airline with a knot or a valve to allow a slow drip or dribble that will take about an hour to quadruple the volume of the bag water.. Exact numbers are not important. 15 minutes will probably work, as will 2 hours.

    When the volume is quadrupled, net the fish and dump into the tank. Avoid transferring any of the old bag water, as you have no idea what free-swimming bacteria or parasites it may contain. Use regular (conditioned) change water to bring the tank back up to full.

    If your water is very soft, and has only 90 ppm of tds, then checking the bag pH may be important, too. Let's say it reads 350 ppm and a pH of 6.2, while your tank pH is 7.6. Any ammonium built up during shipping could convert to deadly ammonia at the higher pH, so again, drip acclimation may be needed. I usually just give the bag a tiny squirt of ammonia grabber, like "Amquel" and dilute the bag water with about 50% tank water a couple of times at 10 minute intervals. I then net the fish and transfer only minimal (skin-wetting) bag water into my tank.

    Just dumping the bag into your tank could give everyone in there a severe case of ammonia burns and that would be coupled with osmotic pressure shock to your new fish. It would likely die and the opportunistic invasion of your other fish would have you cursing the LFS for something that was normally in your tank all along!

    Just as a finger test is plenty good enough to avoid temperature shock, a quick tds reading will help you avoid osmotic pressure shock damage. The tds meter really comes into its own when you do the common salt additives to Nothos, etc. for the evaporation of tank water can make the tds change a lot. As you get experience with your own change water, and the LFS water conditions, you will use the meter less, but saving just one hard-to-get species can more than pay for it.

    Wright
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    Quote Originally Posted by budak
    Would the same idea apply to crustaceans? There is the common complain of shrimps like yamatos succumbing to "pH shock" upon introduction to home tanks.
    You bet. Osmotic regulation is a major part of any aquatic organism's ability to survive. Many shrimps can slowly adapt to nearly any water from soft fresh to hypersaline. Sudden change is a disaster.

    You may have noted that even my Java Moss took a hit from electrolyte imbalance, and shrimp will suffer, too, if there is too much sodium and not enough potassium for proper cell metabolism.

    I have found that iodides and other traces, as well as a bit of Ca and Mg are essential for many crustaceans to be healthy. I usually use iodized salt, despite all the ignoramuses warning against it.

    Premium salt brands use silica gel as the free-flowing agent and it is insoluble, clogging gills and messing up the slime coat. Cheap generic salt uses sodium ferrocyanide, which dissolves and is harmless. They often call it "yellow prussiate of soda" to avoid panic among the chemical illiterates who got their chemistry knowledge from mandatory government schools.

    It is the silica that needs to be avoided, as the iodides are essential to life and at 100X lower concentration than the normal treatments for fish thyroid disease. Iodides and chlorides are relatively non-toxic. Don't confuse them with elemental chlorine or iodine. Both are powerful oxidizing agents and very, very poisonous.

    Some kelps (Nori flakes?) and other ocean algae are rich sources of iodides, BTW. I like some of the better spirulina mixes, like the one from Brine Shrimp Direct at:

    http://brineshrimpdirect.com/brine-s...lina-kelp.html

    I'm not connected to them in any way, just a happy customer.

    Wright
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    Hi Wright,
    Thanks for the detailed and informative explanation. Every piece of post from you has really benefit me a lot.

    I am wondering if the TDS measurement is a important reading for Singapore hobbyist unless he is receiving live fish from oversea. Water of Singapore come from several reservoirs (which is close to each other considering the size of our country) and neighboring country, so I doubt that there will be a great difference in TDS. I might be wrong so could any Singapore hobbyist clarified this? Thanks you very much.

    Regards,
    Ong Poh San

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    Poh San, I'm not a SG aquarist but can't resist answering anyway. :wink:

    Brackish water killies are some of the most beautiful.

    Nothos and some Fp. are sensitive to Velvet and really need some added salt. That radically changes the tds.

    Rainforest fishes have adapted to very high levels of osmotic pressure (i.e., their water is very low in tds) and breeding them sometimes calls for mixing tap and RO water. The advent of cheap RO units and tds meters has made the value of hard tap water go way up. With RO and hard tap water, you can mix and match to suit any kind of fish or invert.

    My water is too soft (50 ppm as CaCO3) 85 ppm total, so I have to add Seachem "Equilibrium" to bring tds up to a less-stressful level for many fishes.

    All of the above processes expand our fishroom capability and would be difficult, indeed, without a tds meter o/e. For brackish (e.g., 50% sea water) I have to use a hygrometer and measure specific gravity, as my TDS-1 only goes to 999 ppm and then gives spooky readings that are easy to misinterpret.

    Wright
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    Bishop, CA 93514 USA

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    Poh San, our water supply comes from different sources so the TDS levels should be different. Try measuring your pH for example, gives you different readings in different areas of Singapore. Most of our water supply is from rivers in Johor State, Malaysia. Some of that is supplemented by our inland catchment reservoirs in the central catchment area, estuarine reservoirs that are actually dammed-up river mouths like the Kranji River and NEWater that's being pumped into the systems from treatment plants. A few years from now we may have treated seawater running in our systems from planned water reclamation plants on the southern islands.

    I would say TDS readings would differ from town to town in Singapore therefore having a TDS meter would be a good investment but not necessarily a must-have. It might help to explain sudden fish deaths when one does a water change.
    Fish.. Simply Irresistable
    Back to Killies... slowly.

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    Poh San,

    here's a link -
    http://www.pub.gov.sg/our_services/W...HowBrought.php -to the page where you will find the table with all the water parameters from the different waterworks.

    You can see that the tds varies.
    Zulkifli

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    Wright, a small group of us ordered the TDS3 and TDS4 meters, received the package today and I proceeded to test-check all the meters.

    Straight out of the product packing, there are minor discrepancies in reading between the range of 071~085 ppm. Water tested is what I'm getting from the mains at work and the discrepany between models is negligible, I think.

    Let me get the boys together for collection and have a go with it before we flood you with further questions :wink:
    I'm back & keeping 'em fingers wet,
    Ronnie Lee

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    Ron,

    Thanks to you and now I'm having fun with my TDS meters. Ok, here goes for the various tds readings for my various setups, comments please:

    1. Tapwater

    75 ppm

    2. My 4 Ft planted tank

    260 ppm

    This is my main display tank, which is abt a yr old, and I've been generous in liquid fertilisation. Substrate fertilisation is present, with CO2 injection. The whole tank is choked full of plants right now.

    Water change is done weekly with tapwater, % volume changed is 50%

    3. Goldfish 2ft tank

    501 ppm

    There is only a single goldfish in here, and NO water change has been conducted for the past 5 months or so. The tank had been setup for almost 8 months.

    4. Apistogramma borelli 2 ft tank

    93 ppm

    This is a 5 month old setup, that started with a pair of the borellis, and now the population is stabilising at about 40, with simultaneous spawns every week, though sibling caniballism is extremely high these days due to overcrowding.

    This tank gets its water change once a month, using tapwater. % water changed is abt 20% each time.

    It is a low tech planted tank with no fertilisation at all, not even substrate fertilisers. The only plants in there are java ferns, anubias and Echi. quadricostatus. Moderately planted.

    5. Nannacara anomala 2 ft tank

    141ppm

    Another simple low tech tank with few plants, only a bunch of Vals and some pieces of driftwood tied with java ferns.

    This tank's inhabitant is only a pair of the dawrf cichlids, and the tank is only 3 weeks old. No water change has been performed as yet, and the water has been soften by a stocking of peat moss.

    No substrate or liquid fertilisation in there.

    Cheers,

    Kenny
    [/b]

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    Hello all,

    Got this post from a friend needing help with TDS:

    I want to ask a favour - would you answer some questions about water for me?

    I've read quite a bit now and understand most of it but I have the following situations.

    I just purchased a Hanna TDS/ph meter - the readings I am getting are as follows:

    I have an RO unit that produces water with a TDS of between 0-20 uS (microsiemens right?)
    This is very soft water - I understand that.

    I recently bought 4 very young discus very cheaply (couldn't resist them at the price) and when I measured the TDS they came in (bags) it was 2690 uS with the same meter!
    Am I reading this right that this is very hard water and not very good for them long term. I thought i would gradually reduce it to under say 300uS but this seems a long way to go.Plan to do this through water changes over some months.
    I have two 4ft tanks that read off the scale with the meter - ie over 3999 uS!. Killies seem fine in the tanks. They are mostly odd males and females/pairs. Should I try to gradually reduce these too?
    Should I aim for under 300uS for soft water killies? I have discovered how to mix the RO water with bottled mineral water to get a specific reading.

    Lastly what is the dh/gh relationship to TDS - if any?

    I bought the meter because a Diapteron keeper I have been talking to says that TDS seems more important than ph. I really want to raise Diapterons but am not going to get any more until I fully understand the water quality required.

    Sorry this a bit of ramble but I think you are interested and knowledgeable enough to help me.
    Wright? Think you can handle it?

    Regards

  20. #20
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    Tyrone:

    Of course I can handle it. The question is, what should I do with it?

    The Diapterons probably will do best if the GH is low. My perception is that they are a species whose eggs don't like to hatch in harder water. I like to keep them at about 100-120 ppm tds, with at least 1/3 of that being a good mix of divalent (Ca, Mg, Fe) salts and the rest monovalent (Na, K,) salts. I like enough carbonate buffering (KH>3) to keep pH stable. Never try to raise tds of RO water with pure salt. That can be deadly without the other electrolytes to permit proper cell metabolism.

    Discus do very well in hard water, but may not choose to breed in it. The discus world is full of myths and superstitions about water hardness, and I have seen stores kill them in large numbers by trying to use pure RO water. Water without any essential electrolytes can be quite deadly, so RO, distilled or DI water needs to be used with great caution. Amazon water may read very low in tds, but has a lot of organic stuff that doesn't ionize, hence doesn't read on a conductivity meter. It seems to sustain life, unlike distilled or RO water of the exact same tds.

    [Simple test: Add sugar or alcohol and note that tds meter does not change. Add salt and it does.]

    Lastly what is the dh/gh relationship to TDS - if any?
    GH is a part of the dissolved solids that add up to the total dissolved solids (tds). Often it is about 2/3 of the tap water solids, but the water report will give you a good clue and you can guess it after that. IME, the ratio of tds to GH stays about the same, even though there may be seasonal variations in those. YMMV.

    My simple rules of thumb:

    100 ppm tds is about right for the softest-water fishes. Fp. and Nothos like 200-500 ppm, as do many estuarine Lampeyes and Aphyos.

    Water below 50 ppm must be brought up with Seachem "Equilibrium" o/e.

    Water much over 1000 ppm is suitable only for brackish-water fishes, usually.

    Don't subject fish to osmotic shock, by suddenly using water more than 1/2 to 2X the tds. This is the source of the pH shock myth, and is far more important. [I subject delicate fish to pH changes of 2-3 points with absolutely no ill effects.]

    Wright
    01 760 872-3995
    805 Valley West Circle
    Bishop, CA 93514 USA

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