Not to be contrary (though that’s exactly what I am doing ) but this is really where increased water changes, optimized oxygen levels, and improved tank maintenance and (perhaps) feeding can be the most impactful actions a fishkeeper can takeNot advocating Melafix or Pimafix are wonder cures but as there is no definite diagnosis and being natural products thought it better than some more invasive meds at this stage.
(note this is not just my opinion but one substantiated by the fine folks at places such as https://aquatic.vetmed.ufl.edu/education/programs/care-conservation-of-aquatic-animals-certificate/
This Aquatic Veterinary college has always offered a great collection of articles for hobbyists and aquaculture industry)
I should mention that I first tried that wonderfully acclaimed Melafix/Pimafix combination when my Altum angels began to display minor skin lesions and less than perfect fins
They began to show stress within a few minutes, and increasingly adverse reactions - several large water changes later the fish were decidedly unhappy (but no longer looking as if they were about to expire)
I did add carbon BUT as these are oil based medications, it’s not particularly effective
I continued to do 80% water changes every 12 h
After a few days, they finally returned to the initial state at which I’d determined I needed to do something despite no clear indications of what was amiss ... it turned out to be an incredibly nasty internal parasite
The skin lesions are where single maturing parasites have set up home, in addition to the hundreds of immature parasites infesting gill tissue and gut - as the mature parasite “digs in” and prepares to (disperse eggs??? mate???) the skin lesions become pronounced and nodular, and then one day, there is a white thread-like-worm extending from the fish and drifting/wiggling in the current ... and then there are more (twining and twisting together)
Except they aren’t actually exposed to water column mediations (hopeful toxins) as there is a very fine film “bubble” protecting the “home” and extraction is difficult as there is a barb-like attachment deep in the muscle tissue
And these are not a few mm’s in length but cm’s (the longest I measured was 4-5cm, though most were 2-3cm, only they remain tightly coiled in at least part of their length)
In the beginning I had this notion that I might just need to remove some external parasites - until I’d performed a necropsy that revealed the masses inside (I’d hoped most of the parasites would be localized to the skin nodules, and that I’d find immature stages there ...)
After several months of levamisole treatment, medicated food and water column, I had a survival rate of ~40%
But it was a year from onset before these fish really looked/acted healthy again.
It was a few years later that I saw some (poor resolution, blurred) photos on Pandora’s Box that showed the same unidentified parasite
Apologies to @shangman for the off topic wander