Hello lovely, poacher turned gamekeeper eh
Happy New Year!
Pat messaged me so here I am …
Hi Filip, have you taken a sample from the fish? Before hitting it with any chemo I'd be tempted to try and nail down what it is your dealing with. If it is a parasite causing these spots then it would be visible under a scope - assuming each spot is a single instance. Have you removed a scale or tow over a spot at all? It generally doesn't do any long term harm. I would look at the scales removed to see if they are discoloured. Also has the fishes behaviour changed? any flicking/flashing type behaviour? (this would be expected with a parasite). I'd also expect the spots to be slightly raised if is blackspot as the spot is a cyst.
If it is blackspot there must be a host or intermediate in the tank - usually snails. So if you remove the host/intermediate, the blackspot will die out. Looking at the vid I can see it has plants and what looks to be at least one snail.
I would expect praziquantel to work well if it is blackspot. You could also isolate the fish in a qt tank for a few weeks and see if that changes anything. Note that the discolouration is melanin I think so may not go away or go away slowly. It is also possible that what you are seeing now are the after effects and there is no active disease - if the other fish are free of these spots and no further spots are developing then it suggest there are no active parasites.
Hope that helps.