| | Re: CO2 system suggestions for a 75gl tank
If you stop injecting CO2 at night, or any other time, the amount dissolved in the water will drop, possibly slowly, depending on how much surface disturbance you have. If the pH controller really does maintain a near constant pH, and I have no doubt that it does, then it has to allow some CO2 to flow periodically at night as well as during the lights on time. Every morning my drop checker shows a higher pH than it showed during the previous day, so I know my tank, at least, does lose CO2 at night. I purposely keep some surface disturbance to increase oxygen absorption and to deplete the CO2 at night a little quicker.
In a fish only tank there is no CO2 injection to make the pH change. In fact the pH should remain virtually constant unless something generated in the tank makes it change. And, if that happens, the substance making it change is adding to the TDS, and may even be harmful itself to the fish. So, for a fish only tank maintaining a steady pH is desirable. It is easy and cheap to measure pH, compared to measuring TDS or a concentration of "bad" substances, so for many years it has been recommended to maintain a constant and sometimes even a specific pH in a fish only tank.