Tough to see clearly through the algae on the glass. This is a toss up between late stage ich and Brooklynella. I can see distinct spots, and that points to ich, but the stilted way the clownfish is swimming leads more towards Brooklynella. Are there any other fish in the tank? Are they showing any symptoms?
Ich is best treated with coppersafe in a treatment tank (no invertebrates). Brooklynella is best treated with formalin in a treatment tank (as both a dip and a static bath). Formalin is tricky to use and difficult to source fast enough.
If you don't have a treatment tank, you're pretty limited. If this is Brooklynella, you *might* have some success with Ruby Reef Rally Pro as a dip and a bath (mostly reef safe).
If it is ich, you really won't have a way to treat this in-tank.
Jay
