It sounds to me like the tank wasn't cycled. That would definitely stress the fish enough to cause slime coat problems. Ammonia and nitrites need to read 0, Nitrates need to have a reading, that you are reducing with regular, partial water changes. Then you know the tank is stabilized. The addition of salt at a teaspoon per net volume of the tank can sometimes help reduce the toxicity of nitrites, but personally, I would do a 30% water change a day until they were at 0 and you are showing nitrates.