try doing some daily partial (30-50%) water changes and gravel cleans for a week. then if it hasn't cleared up try adding some medication.
Tetracycline is a very strong antibiotic and will wipe out the good filter bacteria. You will also need a vet prescription to obtain it.
Most normal aquarium medications should clear up a minor infection without the need to use Tetracycline based drugs. If you want to treat the fish then look for Waterlie Myxazin" or "Wardley's Promethyasul" or "Triple Sulpha".
Melafix might even work too and is a lot safer than the other drugs.