How big is the airstone?
Airstones come in a range of sizes but if you only have a small tank, you only need a small airstone.
If there is too much water movement caused by the airstone, the fish will be constantly swimming into currents and that eventually stresses them and can kill them. The fish need calm areas where they can sit and not be washed around the tank. If you don't have any calm areas, then get a smaller airstone or turn the pump down.