Think you will find it's more to do with the Difference in the PH, rather than the Ammonia.
If you put a Fish into Water that is a very different PH level to what they have been in, they will get PH Burn, their Fins will turn White and Basically fall off in time then Regrow.
I would highly recommend Discusguru's advice, do 100% Water Change, but make sure the new Water you are replacing the Old with is in a similar PH range to the Water that was already in the Tank.
I would also highly recommend you get your self a Test Kit, all Fish Keepers should have one of these, far easier to Test the Water yourself rather than having to run to the Petshop to get it tested.