Loaches are mostly fish from upland streams, so they're usually found a long way from brackish water. So far as I know, no loach is an exclusively brackish water fish. But with that said, at least some species do occur in brackish water in parts of their range. Among the traded species, the common horseface loach Acantopsis choirorhynchos is the best bet. It will thrive at 10% seawater, SG 1.003 at 25 degrees C, and as such would be a fine companion for low-end brackish species such as chromides, knight gobies and mollies. But do be careful when shopping because there's a lookalike species, Acantopsis octoactinotos, which is much less tolerant of tankmates.
I'm aware of statements about clown loaches entering seawater. Bob Fenner for example believes this to be the case. He's one of the world's best known marine aquarists, as well as a personal friend, so I'm loathe to dismiss his opinion out of hand. I also remember reading about clown loaches in a marine aquarium book maybe 25 years ago, but can't for the life of me remember the book. To be honest, I've tended to assume statements about clown loaches in the sea are misidentifications or mistranslations. But with that said, clown loaches tolerate some salinity in the aquarium quite well, and it's a much safer way to treat whitespot for example, than the usual use of copper-based medications.
Whether fish have or do not have scales is immaterial to their tolerance of salinity. There are plenty of marine fish that don't have scales, such as sharks and moray eels, as well as fish without scales, such as catfish, that enter brackish water or the sea. Indeed, there are at least two catfish families that are primarily marine in distribution, and several other families that include species that enter seawater, including species from the bagrid, pangasiid, and banjo catfish families.
Cheers, Neale
