It's worry some that you're trying to turn off the c02 by hand every night and on every morning? That's more work than owning a dog lol. Open up the wallet and get a timer (and solenoid if you don't have one) and have c02 as one of those "set it and forget" setups.
But like others said, the real problem is probably your c02 is too high if it's killing your fish at night when plants are producing oxygen. There's no reason for it to be that high; it should be able to run 24/7 without killing them. I prefer to save the c02 so I time it so it's on when the lights are on, but even if I run it 24/7 at that level my fish are fine. (Shrimp on the other hand I have in a low tech tank)