If the tank is cycled then the nitrifying bacteria that consumes the nitrite will continuously create nitrate as a byproduct.
To me it sounds like there could be some kind of an ammonia/nitrite spike however you said your readings were all zero...
It would take quite a bit of nitrate to cause the fish to gasp for air and sicken, but its possible. Do you do regular water changes? If not, then the nitrate will build up to unsafe levels.
Also, how warm is the water and do you use an airstone? Perhaps there is a a low level of dissolved oxygen in the water.
To me it sounds like there could be some kind of an ammonia/nitrite spike however you said your readings were all zero...
It would take quite a bit of nitrate to cause the fish to gasp for air and sicken, but its possible. Do you do regular water changes? If not, then the nitrate will build up to unsafe levels.
Also, how warm is the water and do you use an airstone? Perhaps there is a a low level of dissolved oxygen in the water.
