What sized tank?
Is it filtered?
Did you cycle it prior to adding fish?
How many fish in the tank?
What is the fish's behaviour (laying on the gravel, gulping for air at the surface, darting around the tank, floating upside down etc)?
From reading your question, I would first suspect bad water quality from high levels of Ammonia. That is most plausible considering your fish seems to have inflamed gills in which Ammonia attacks.
Please answer the questions to help us help you.
