How much CO2 is enough?

[bttay]

My tank has a pH of 6.8 and KH of 4.
I refer to the CO2 content table it is 23 ppm.

But sometimes when the pH swings to 7.2 (after water change) and KH remains as 4, the CO2 is 7.5 ppm.

I read some books say that CO2 should be between 7 and 15 ppm. But some say should be 20 to 30 ppm.

Which one is the correct one? Why they differ so much?

Another question is that most of the time the water remains at pH6.8 and KH 4 regardless of whether light is on or off. Is this normal? When the light is off, shouldn't the pH increase as plants do not take in CO2? If you want to know which filter I use; I have a wet-dry filter set to minimum flow and it is running all the time with porous stones and active carbon in it.

[David]

Hi Bttay

I dont mean any punt but I think you are getting confuse regarding KH, PH, and CO2 and how they are inter-related.

PH - Is an indication of the acidity or alkalinity of a given volume of water.

KH - Is basically the amount of carbonate present in a given volume of water that determines how much dissolved CO2 it is able to buffer before a downward shift in PH values.

CO2 - Is the delivery element to provide carbon that plants need to grow. CO2 readily dissolves in water and is one of the major determinate of PH level in a given volume of water.

Back to your question, do not confuse yourself with dissolved CO2 ppm level. You can get into this later when you have more experience should you really want to get that far. As for myself, I opted to use the chart supplied with Dennerle's PH Controller. At a KH of 4, you should be having a reading of 6.5 to 6.6 PH. At your stated reading of 6.8 PH, you are alittle short of dissolved CO2.

What is the amount in terms of percentage do you execute water change?...This swing that you mention is quite normal with large amount of
water change as there is very little dissolved CO2 present. Coupled that you have already a low amount, thus the swing (my guess that is....).

You should be very happy that you can keep a steady PH level reading lights on and off....This is something I cannot achieve till today......(embrassed......[] )......When the lights are off, if you are still injecting CO2, PH will come down, not up. If you have stop injection, PH will come up, not down...

Hope all this blabbering bulls*it helps you......hehehehehe

[bttay]

Thanks a lot David,

You are right, I have no idea about how KH works with pH in relation to the Co2 content[] . I must do further reading and experiment then.

As you mentioned, my CO2 is a bit low. I think I am going to do something to my wet-dry filter. I heard that wet-dry filter is no good for keeping CO2 in the tank.

Thanks again.

[David]

No Problem

I wasnt born with the knowledge and had to go through some very painful experience.....

Dont junk the W&D filter.....it is fine to use them ......just need to expend a little more CO2 which is fine.