Ammonia burn

In terms of skin conditions?

Free ammonia is highly toxic to aquatic life. It kills in aquariums at very low amounts. Any level above 0.02 mg/l (ppm) is considered harmful.

Ammonium may be toxic to marine fish, especially if the pH differs significantly from natural seawater (e.g. during shipping)

Free ammonia causes gill damage, internal organ damage and eventually skin damage and death. Typical symptoms include:
ragged or frayed fins
cloudy eyes
rapid gilling
lack of appetite
more susceptible to disease

In general, ammonia is more toxic at higher alkaline pH values and as the temperature of the water increases.

Ammonia is not toxic to plants in levels that would cause distress to fish, indeed of those plants tested, most preferred ammonia or ammonium as a food to nitrate.

Rough guide to toxic levels of free ammonia:
0.020 to 0.049 (ppm) is considered 'tolerated' but will cause long term harm to its growth, immune system, health, etc. especially to eggs or very young animals.
0.050 to 0.199 (ppm) is perhaps tolerated for only a few days and is very harmful.
0.200 to 0.499 (ppm) is perhaps tolerated for a day or two and will probably kill.
0.500+ (ppm) is deadly and will probably kill within a day.
Individual species of fish, amphibians, invertebrates etc. vary enormously on their tolerances of low levels of ammonia and the issue is made further complicated as young are far more susceptible to ammonia than older animals.
In nitrogen sensitive fish like Trout, ammonia is about 6x more toxic than nitrite and about 13,300x more toxic than nitrate

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One thing you should check is if you are dealing with An ammonia burn or with velvet developing from a previous infection of ick or similar infection that can lead to stress.

 

Did you test for ammonia, or are you just guessing?

 

There is your answer.

How long has it been set up, what kind of filtration is on it?

 

Go get yourself some Seachem Prime asap. It wont remove the ammonia but it will help mitigate the toxic effects.