He's asking about the outside of a saltwater tank... So I think a pleco is out of the question, lol.
I use a new kitchen sponge with a scrubby side and a mix of vinegar and water, then when I'm done I go over it with a wet paper towel then dry it with a dry one. Works pretty well, but I have a leaky top rim so I have constant salt creep down one side of my front glass. It drives me nuts but I'm putting off resealing since I don't have a place to put my puffers for three days to reseal the rim. Might do it when I move, though.