I have two goldfish in a 10 gal tank. One keeps coming to the surface for air.?

I have both an aerator and a filter. I was reading online that when a goldfish is constantly piping, it means that the tank doesn't have enough oxygen. Also, this tank is relatively new, and the ammonia and nitrites were high, according to water testing done at Petsmart. We did a 10% water change last night (which a PetSmart clerk told us to start doing daily), added AmmoLock, Stress Zyme (to begin the biological filtration). I am guessing that the high levels of ammonia and the fact that the biological filtration takes a few weeks to work is the reason for the piping. Is there anything more I can do to help my fish? Can we increase the daily percentage of water change from 10% to 15% or 20%. If you do a daily water change, what is the maximum percentage of change that you can safely do? We had a goldfish that was doing the same thing, and the next day I found him dead in the tank. I don't want this to happen again.