Does the OP have chlorine or chloramine in their tap water?
If you only have chlorine, you can fill a bucket with water, put it out in the sun and aerate it vigorously for an hour to remove most if not all of the chlorine.
Chloramine needs a dechlorinator.
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If the fish are gasping at the surface, there is not enough oxygen in the water or something is affecting their gills preventing them from being able to get oxygen from the water. If there is something in the water affecting their gills, all the aeration in the world won't help.
Aquarium test kits only test for ammonia, nitrite, nitrate, pH, GH & KH. Any other chemicals in the water won't be picked up by aquarium test kits so your water might look alright on a fish test kit, but if fish are gasping at the surface, there is a problem.
Water changes and carbon are the quickest and best solution to poisoning in fish tanks.