Depending on how you feel about the drop checker failing to work, it may be giving you the answer.
Drop checkers are pretty simple items but the fluid used can be a real question if we don't know for sure that it is mixed right. Depending on commercial mixes can make us doubt the results so I do my own. If you have 1/8 teaspoon measures, sodium bicarbonate and distilled water, it is cheap to make. Recipe is on hand if you need a copy?
My way to find what is going on would be to operate on both issues. First I would get a drop checker and fluid that I felt was right. I would turn the input line down so that the Co2 is going into the water. Do this at some time when you are going to be home to watch the fish. Don't increase CO2 without watching the fish. If they begin to act strange in any way like going to the surface, less activity or not eating normally, back off the CO2 and do a bit of water change to help them. Take about a week or more of slowly increasing CO2 and at some point you will reach a point where the drop checker changes color or the fish no longer adapt to the new amount. At that point you will know you are at the max for your tank.
Turning the input up seems like a pretty weird way to go about limiting the amount you are getting in the water! Most use a needle valve to do that. Kind of like jacking the car up to limit speeding tickets? Works but it is missing some of the finer points.
Last edited by PlantedRich; 08-13-2014 at 07:22 PM.