Blister?
Hi Grey, Good to see your postings again, but not necessarily under these circumstances. While I’ve never seen anything quite like this on Angelfish, mainly as these abnormalities are rare, I have seen similar parasite-caused cysts caused by a trematatode (Clinostomum complanatum and/or C. marginatum) mostly in wild fish in which this cyst is the intermediate (larval) stage between its egg which infects a snail species (of the Genus Helisoma) and a fish-eating bird (Heron, for instance) in which it reaches its adult stage. This usually doesn’t affect the fish in the least; it may have been originally picked up at a Florida fish farm if this is where the fish was reared, but you don’t say how long its been like this although it seems to be rather sudden.
It can also be caused by a number of other parasites of protozoan origin such as Cyclochaetiasis, Chilodontiasis and Costiasis. Costia is the most dangerous of these three and while salt treatments are often effective against the first two, treatment for this disease requires a formalin solution for best results. Unfortunately, they cannot be told apart except under a microscope. Then too, these blisters can be caused by a disease known as Lymphocystis (bacterial), although these grow slowly and would have been seen much earlier.
Viral infection may be a cause and cannot be ruled out, so it’s really difficult to diagnose. From the appearance of this, it looks to possibly be tumorous, rather than any of the previous pathogens and noting its position out beyond the confines of much other tissue, I see no reason why a quick and careful snip with a nail clipper would not entirely remove it intact (to be retrieved and disposed of) without affecting the integrity of the dorsal fin. A topical application of an antibiotic, such as mercurochrome if available, should then be administered. Ray
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