How big is the tank in gallons, what fish and how many, is the tank cycling, what is your ammonia and nitrite reading, and are the fish gasping at the top of the tank.
How long has your tank been set up? The ammonia levels will drop by being processed as part of the nitrogen cycle. I'm guessing you have fish in there and if so you will have to perform daily water changes to try and get the ammonia level down to protext your fish from exposure to the ammonia.