Whilst some say higher levels CO2 do not reduce dissolved oxygen in the water (it sometimes can, depending on other factors) even moderate increases can have some fish gasping at the water surface. If it doesn't affect the dissolved O2 then what's going on?
To diffuse the CO2 out of their blood across their gill surfaces, fish require a concentration differential between the water and the blood. As the differential diminishes due to increased CO2, the gas exchange slows and stops. The fish can't excrete the CO2 and they die. This is sometimes exacerbated by aquarists 'turning off' the air stone so as to reduce CO2 loss at the surface. This the results in lowering the dissolved O2. Then the lights go out and the plants start respiration. They stop eating CO2, start using oxygen and the balance tips and in the morning you find some dead fish and it's a mystery.