Vets don't treat fish.
How is your fish sick? We need a LOT more details to diagnose the problem! What species of fish is it? What is your tank set up and how long have you had the fish?
If it's acting up the first thing you need to assume is the water has gone bad and to test that and perform a partial water change. Virtually all of the disorders that fish suffer from have been primarily caused by bad water quality due to poor filtration/uncycled tanks/undersized tanks/lack of tank maintenance.
