Most Fish Died?

[hogan53]

Co2 is mostly benefical for a well stocked planted tank as the plants take in Co2 when the aquarium is lighted properly and give off Oxygen. This helps plant growth.
Co2 must be shut down at night time or when lights are off as they also take in Oxygen and expire Co2
which can have a drastic effect on fish,that is why you must use a timer to shut down the Co2 and have some sort of aeration when lights are off, also stocking levels cant be as high in a Co2 set up.