Firstly if you don't have a skimmer or adequate water flow your oxygen lvls are probably too low.
Secondly while certain parasites do show up on the body of a fish like ich they can take up residence in the gills as well which will also cause breathing issues.
Lastly a dwarf angel is probably the last fish you add to a tank as they tend to be the most aggressive, and 2 tangs in a 90 gallon isn't really recommended.
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