Whats my neon tetra doing?

ok so i have 16 neon tetras in my tank. well one of them keeps trying to swim to the surface but seems to be struggling. what is it doing? i have an airstone in the tank and my ammonia and nitrite are both at 0. its the only one thats doing it, its had some of its tail fin bitten off by one of my serpaes, but some of my poor neons have no tail fin and they still manage to swim.