There are many reasons that fish may have black spots,,depending on the species of fish,,and the genetic line it comes from there may be black spots from pigment alone, and not nescessarily posing a health risk. Bettas come to mind,,many strains of bettas have black spots around the head region.
Some wild types of fish like silvr dollars, pacus, and even piranhas will have a parasite whose larvae appear as black spots. Cercaria sp come to mind. These require snails and even birds to carry out their life cycle and in the absence of these the parasite cannot carry out it's life cycle.
You didn't make any comments about what is specifically going on in your tank so it is hard to give precise advice, but I can say that you should not add any medications until you've diagnosed specifically what you have,,and as long as the wate quality is perfect then I'd be skeptical about adding anything. Bill