Ammonia in your Tank
When a tank is first cycling the ammonia levels can get quite high and by high I mean anything above the 0ppm mark. Anything higher than that can cause burns on the fish's fins, some times death. When the ammonia level is above the 0ppm its better to let the tank cycle longer by doing small water changes everyday and testing the water to see were the level is and if you need to do bigger ones. The normal size of a water change when ammonia is present is about 20% at a time. If the level is in the high range of lets say 3ppms I would do a 30-40% water change to give the goldfish the relief of the ammonia in the tank. You can use stuff like Amquel to neutralize the ammonia but the problem with that is if you dont have the right test kit with the Amquel you can get fake readings. Also the Amquel doesnt remove the ammonia from the tank it just makes it less toxic to the fish.
Other problems with the ammonia is also your Ph level. If your Ph level in the tank is above the 7.2 range, your ammonia is actually in a sense more toxic to the fish. So when your cycling its good to know what the Ph level is in the tank cause if your Ph is a high level and your ammonia is at the 2ppm then in a since you could look at the ammonia being at the 4ppm and this can be deadly to the fish.
Now when you see a fish that has Black edges on the fish some of the times this is do to the ammonia burning the fish and is in the healing end of it. Nothing to worry about just keep the levels in the tank in a clean environment.
Always remember water changes are always good for you fish.