| | Re: CO2 system suggestions for a 75gl tank
The reason for using a drop checker is to isolate the water used to determine the ppm of CO2 from anything in the tank water, other than the CO2. The tiny amount of water in the drop checker has to be free of the ions that would affect the pH or KH, except for carbonates, which bring that water to a 4 dKH hardness (alkalinity).
I have never found a way to determine how "contaminated" the tank water is, and how it will affect the pH or KH. It must be possible to do that, if you have the test equipment needed. I do know that every person I have heard from, either by comments in forums or in person, who measures their CO2 concentration by using the pH/KH/CO2 table always gets a number that is too high, never too low. Typically I hear that someone keeps his tank at 120 ppm of CO2 and the fish are happy as a lark. That just isn't possible. I know from my experience that a ppm of CO2, based on a drop checker, that makes the checker really yellow will kill even guppies, and that would be somewhere in the neighborhood of 100 ppm of CO2. It isn't known, to my knowledge, how much concentration of CO2 kills which fish, but that number is almost certainly less than 100 ppm. The most recent information I have is from Tom Barr, using a precision CO2 concentration meter, who found that concentrations around 40-50+ ppm don't bother very small fish, but they do bother large fish. I think this is why so many of us have found that keeping our drop checker in the yellow-green area doesn't bother our small fish.