Hi I'm new to this forum and to aquariums. I have a 15 gallon high tank. It's been running for about 2 years. Lately my Cory catfish has been gulping air at the surface. So I figure I need to add an air stone. I just ordered a sponge filter and stone that should arrive in a few days. The tank has a hang on filter now. My question is should I run both filters or just the sponge filter? There's only 5 fish in the tank , swordfish, molly, tetra, peco and the cory. You're help is appreciated