What's the best way to tune CO2 rate? - The Planted Tank Forum
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post #1 of 1 (permalink) Old 09-08-2019, 08:25 PM Thread Starter
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What's the best way to tune CO2 rate?

I thought I was being smart by using the pH/KH relationship to tune my CO2 injection but I am having a hard time figuring out the right bubble rate and lead time.

My tank is 4 dKH and per the pH/KH scale, at pH 6.60 I should have 30 PPM CO2. (Though, other things in the tank can depress pH, so actual CO2 level may be somewhat less than what is suggested by the pH. You shouldn't have MORE CO2 than the ph/KH chart value... right?)

Yesterday I though I had it pretty good, the pH at lights on was 6.67, and it dropped to 6.45 over the day cycle. 6.45 is lower than desired, though the fish looked OK, so I thought I would just turn off the CO2 an extra hour or two before lights off so I hit a minimum of about 6.5.

So far so good, but you know what they say about the man who has 2 watches... he's never sure what time it is.

Last night after collecting this data, I installed a drop checker. They take a long time to equilibrate, so I left it and went to bed. This morning, the drop checker was green and the tank pH was 7.0. This implies that that I have enough CO2 somewhat before I looked at the DC, when the water was probably around pH 6.9 - 6.8.

That also means that at pH 6.6 in the previous day cycle, I had way too much CO2 even if the chart said I was cool. And, this morning, my catfish is lethargic, which also suggests a CO2 overdose.

Given this scenario, how would you go about determining the right CO2 rate?

I guess my plan is to keep the gas off until tomorrow and let it all blow off, then turn down the bubble rate and see if I can get the DC to be green when the lights come on, which I hypothesize will happen at a pH above the chart's 6.6. I can use the pH to monitor stability of CO2 levels even if I do not trust how the pH maps to CO2 PPM.

Any other thoughts? Many thanks in advance.

More info:
  • Tank size is listed at 90 gal, true water volume is about 75 gal.
  • Substrate is fired clay gravel (Turface) which is now VERY old and should not impact KH/pH as it does when new
  • CO2 reactor is a large Max Mix, it should have more capacity than needed for this tank
  • Tank water and drop checker were both verified as 4 dKH
  • Pinpoint pH probe is new and was recently calibrated successfully
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