Also, Formalin can work with velvet too. It is not a 'gentle' med; but brook & velvet are not gentle parasites. Assuming it is one of these; it may be unrealistic to think you van save all of your fish. Not trying to make things worse, just a warning.
Edit: The above did't post right away, no idea why.
The product you have has used successfully by several folks on this forum. It's unusual for brook or velvet to show up without a fish bringing it in. Anything wet; LR,corals, etc, could bring it along; but that would be rare.
Is this fish eating, gasping for breath, scratching on rock or substrate, etc.? Again, any skin peeling or sloughing? This looks exactly like brook, but if it isn't showing up on other fish, I just can't be sure. Clowns are the most frequent carriers of brook. Some folks think tank-bred clowns, like yours are immune; but they are not, just less likely to have it than wild clowns. If the fish doesn't have any other symptoms, then I'm just guessing. I guess anemone or serious coral stings are possible. Fish with brook or velvet look and act very "sick". If this is still the only fish showing symptoms, you may want to just be ready with a HT. Just watching the all the fish closely goes against everything I know about treating parasites; but this could be something else. As long as no other fish are showing any signs of a disease/parasite; they won't be hurt any more if you just watch a while. If it is brook or velvet, it will show up on other fish soon.
