pH queston - 6.0 and 3 dH - CO2 of 90ppm!

[Rick Dotson]

I calibrated my pH meter to find a pH 6.0 and then checked my KH with Red Sea kit and got 3 KH. I checked the readings several ways and with other methods. This would give a reading of 90ppm of CO2 which can't be right because my fish all look healthy and active. This was also the night time pH, when the lights and CO2 came on the pH dropped to 5.8, the daily water change brought it up to 5.9 for a couple hours before returning to 5.84.

I searched the web and couldn't find anything conclusive. The only thing I could think of would be some humic acid buffering component from the build up of 10 years of stuff in the gravel. Does anyone have any ideas?

[RTRJR]

Acids distort CO2 readings. The easy test is to take tank water and aerate (airstone) it for 20-30 min and take the pH again. The CO2 blows off and no longer depresses the pH. Most organic acids are non-voiatle. The difference in aerated and non-aerated water samples is a more accurate check on CO2 levels..

I don't know why this is not more widely known or discussed on the web. Most established tanks have at least some acids in the water other than dissolved CO2.

[Rick Dotson]

Thanks, I am wondering if I should try to buffer with Bicarbonate or something to raise pH to 6.4 or something? The fish seem healthy so I wonder if it is a problem.

[Aaron]

If it isn't broken don't fix it. If only I could live by this mantra myself more often.

Seriously though, if the fish are fine I wouldn't worry about it. What sort of bubble rate are you using on the tank and what size tank is it?

If the fish are blackwater species such as Rasboras or Tetras they should actually do better in the lower pH.

[Rick Dotson]

Rasboras, clown loaches, gold barbs

70gal, I have never been able to keep decent CO2 level due to off-gassing. So I don't think the CO2 is higher than 20ppm.