Re-test the ammonia and make sure you're using a good quality test kit. The test tube and liquid type are the best.
Then: do a partial water change, (30%), using a good quality declorinator: one that removes ammonia and detoxifies nitrite.
Then: check your filters to make sure they are running properly. There should be a good strong flow coming out of the return. Open them up and make sure there's isn't a build up of gunk and waste on the media. If there is rinse them off in a bucket of tank water.
Then: post back with answers to these questions:
Did the fish show any change after you did the 30% water change?
How long has the tank been set up?
How long have you had these particular fish?
How many fish and what size tank? What species of fish are these?
After you do the water change add salt, (aqurium salt or table salt with NO additives) at the rate of 1-2 teaspoons per ten gallons. It will help detoxify any nitrite.