GH, KH, PH --> CO2

Yildirim,

At KH of 4-5 it doesn't take much to lower the pH, but your tank obviously has some issues.

It is strange that the fish start to act as if suffocating when you increase the CO2. Maybe you can try to increase the CO2 slower - over a period of 2-3 days.

Also, your water may somewhow contain a pH buffer (some substrates for example come packed in buffered water). In that case it takes a lot of CO2 to lower the pH. Considerable amount of CO2 leads to a minor pH change. The CO2 is in there but you can't read it using the KH/pH table, and you pump much more than needed.

You say that the plants start to pearl heavily when you manage to increase the pH to 5-6. From my experience with such low CO2 plants don't pearl heavily and fish won't suffocate. It seems like you may have a pH buffer in the water but if you never added it (intentionally or not) then I don't know what to say .

Another thing to look at is your surface agitation. Maybe the surface of your water is being stirred too much by a powerhead or a pump? That leads to a lot of CO2 being lost. But the fish won't suffocate in that case.

Lastly - your GH is way high. The GH is a measure of the Ca and Mg, and most likely you have too much Ca or Mg, and none of the other. Such imballance does stress the fish (I've seen it only once though). If your tap water has the same or close GH as your tank does changing the Ca:Mg ratio is possible only with adding RO or distilled water. For a start test the Ca using a Ca test kit if you can buy one. A ratio of 4:1 Ca:Mg would be desirable.

--Nikolay

Thanks for your valuable comments Niko. What Ican tell you are;

1. I tried to raise the co2 bubling partialy in 2 days, not at once.
2. I don't use a special substrat. What I use is 6-7cm of 0.5-2mm naturaly colored (mostly black) sand. I do not know if that black sand may release magnesium or not.
3. I will have a better test kit from SERA next month, then I may have some readings for these mg, ca...
4. I suppose there is a misunderstanding (if not mistyping) that the ph value doesn't change to 5-6 but co2 readings from table goes up to 5-6ppm.PH change is as low as 0.1-0.2 degrees.
5. There is a very slow surface agitation as the output of my canister filter is 20-25 cm deep attached to the middle of the back wall and pumps trough the sides of the tank, not to below or surface, which I intensionaly made to prevent valuable CO2 loss.
6. I currently didn't start to add any nutrients or chemicals to the water as it is a new setup, just 3ml of Tetra Flourish to add some traces including K.

Thanks again,
YILDIRIM