40ppm of CO2 won't be hazardous to your fish. The easiest check to do is observe the fish just before the lights go on. If they're swimming/acting normal then you have nothing to worry about. If you see them gasping for air at the surface then you should hook up an air pump to a timer to run during lights out. This will outgas excess CO2 when the plants aren't using it. I would have the pump shut off a couple of hours before the lights turn on so that the CO2 can start building back up for when the plants need it. |