I am sorry but petsmart uses strips which are inaccurate at best. They will also tell you your water is 'safe' or 'fine' when its actually not. According to them, .50ppm of ammonia or nitrite is 'safe' for fish. Quite the opposite! Tetra med will also likely have affected any good bacteria you may have had in this tank which means your water conditions are likely poor right now.
At this point, if you honestly want to help your fish, seek out the aid of an aquatic vet. They have surgical interventions and more approriate meds that can be tried. If you need help finding one, just ask!