ciliated protozoa, i hate them, well the nasty ones anyway - uronema, brooklynella hostilis, i'll even throw that nasty dinoflagellate, amyloodinium ocellatum, in there. as others have said, ciliated protozoa are what most attribute weak snick to, fresh water dip (pH & temp matched) does offer some relief. formalin baths can be done as well but i prefer FW dips. instructions for both can be found at both seahorse forums.
how is his breathing? they will sometimes infect the gills as well resulting in deep, labored breathing, sometimes huffing. have you seen him snick at nothing? i mean, literally, just snick like he's trying to clear his snout?
also, try and get something other than brine shrimp for him.