The smaller the tank the more you dose?

[geoffrey]

Hi All,

Not too long ago, I had the privilege discussing planted aquarium with Vinz at Teo's place. Nice chap! : ) He mentioned to me that bigger tank are usually more stable than smaller tank. That statement puzzles me then. Is it really so?

That has always been in my mind and it is only recently that I came up with a rational explanation which I think will support the claim I too agree. Here's how I see it -

Assuming we have a 1 litre tank with a sword-plant and another 100
litres tank with the same sword-plant. Assume both tanks are NO3
limiting. Assume we follow with a dose of 5~10ppm NO3 into those tanks

Now, will that be enough for the 1 litre tank? No IMO, dosing that amount will only give max, 10mg in total of NO3 to the 1 litre. However, the same
dose will give 1000mg in total of NO3 to the 100 litres tank. Will 10mg in the small be enough to sustain growth for the Sword? Which is better? Are we overdosing in the bigger tank?

But we may say that a big tank will allow you to plant more. No again IMO. The marginal increase in water volume is much bigger than the marginal increase in plants.

Is that why bigger tanks are more stable when compared to smaller one? I certainly think so. What are your views?

Consumption will appear to be fast, particularly in smaller tank but may not appear so in bigger tank hence, most of the time we are not able to detect them and are confused when we compare with test kit measurements.

Based on the example above, assume the sword-plants uptake 10mg a day.
In the smaller tank, we will immediately measure zero NO3 the next day. But the bigger tank with 990mg of NO3 left will probably still show you 10ppm of NO3 with your test kit although it is 9.9ppm. The sword-plant in the 100 litres tank will last 100 days before you need to dose NO3 again but not your 1 litre tank! And by the 50th day will we only detect a drop of NO3 from 10ppm to 5ppm. Is that why most of us are questioning the accuracy of our test kits?

We tend to focus too much on concentration and forget about the total
amount available to plants. 10mg of NO3 clearly isn't enough for plants in the above example. Hence, should we dose more in smaller tank so as to create a reserviour effect inherent in a bigger one?

[vinz]

Interesting argument... I've no concrete answers to this, but here's my take.

There is a flaw in the later part of your theory. There is a reason we measure by ppm and not by absolute amounts. Plants need a certain concentration of each nutrient in the water to be able to absorb the nutrients via osmosis or similar process (any knowledgeble ppl can enlighten us with the actual process?)

So, yes, in a smaller tank, the NO3 levels drop much faster and you must dose more often to maintain the required concentration levels. Thereotically, you can dose more to create a reservoir effect, however experience in the hobby indicate that too high levels of NO3 and/or PO4 can contribute to algae problems. (Some forumers have had no major algae problems though with levels much higher then recommended)

There is also the concept of "critical concentration" or something like that. I think FC can shed more light on that. From my limited understanding, that's the minimum level of a nutrient's concentration a plant requires to be able to absorb the nutrient from the water. So while you would raise the NO3 levels to 10ppm after a water change, you can let it drop to about 5ppm before topping up to 10ppm again.

The general explanation given to explain why large tanks are more stable is their ability to buffer changes/errors/neglect better. You partially explained it in your argument. Nutrient levels probably maintain their levels better/longer in large tanks.

Consider also dosing variance. Assume you dose 5ml per 100l using a measuring cup and an approximation error of +/- 1ml. For a 100l tank, that's an error of +/-20%. For a 500l (25ml) and 1000l (50ml) tank, it's 4% and 2% respectively. This applies to dosing everything from baking soda (to raise KH) to CO2.

Large tanks also buffer temperature changes better. Temperature in a large tank takes a longer time to rise and drop. Temperature in different parts of a large tank probably varies more between areas, which may explain why some shrimps can take 30 celcius in a large tank better then in a small tank (i.e. shrimps in the large tank simply retire to a cooler parts of the tank, while in a small tanks, there are no significant cooler part to escape to).

If we look at the actual numbers, some ppl may conclude that the absolute differences may not make a big enough difference to affect the stability of the tanks. (E.g. so what if I raise NO3 to between 8 to 12ppm in a 100l tank vs 9.6 to 10.4ppm in a 500l tank with each dose? Does 2ppm make that much difference?) Perhaps it's the frequency combined with the size of the variance that makes a difference?

[wynx]

Vinz,

No expert here. But I do agree with what you stated!

The general explanation given to explain why large tanks are more stable is their ability to buffer changes/errors/neglect better. You partially explained it in your argument. Nutrient levels probably maintain their levels better/longer in large tanks.

When we talked about nitrate buildup in a tank due to fishload of 100 tetras in a 2 foot tank versus one four tank with same amount of fishload. The nitrate buildup can be quite significant on the 2 footer and not on the four footer. I am from the school of though that the bigger the tank, the more forgiving it is to the aquarist. Well, the stability of the tank improves with size. Think of pouring a whole bottle of TMG into singapore river...What is the net effect? Versus a whole bottle of TMG into your aquarium..u will see what happened.

Just my two sens...

[geoffrey]

Vinz, very good points. Oh yeah, the osmosis and ppm part reminds me now on the experiment we did in school with potatoes and a concentrated solution in a beaker. How can I forget?!?