There are multiple catfish well suited to slightly brackish water conditions, and at least 4, possibly 5, families include members fully adapted to saltwater conditions.
Anyway, of the traded species, the best "scavenger" is Hoplosternum littorale, a medium-sized relative of Corydoras. It is gregarious but boisterous, and does well up to SG 1.005, making it a good choice for use alongside low salinity species such as livebearers, sleeper gobies, etc. Some of the Corydoras from the coastal rivers of northeastern South America may tolerate slightly brackish conditions, but I've no idea which ones, and doubt they're traded: many of the species from that part of the continent are rather drab and unexciting.
Rather few loricariids are naturally found in brackish water, though a few are, notably various species of Hypostomus such as Hypostomus watwata. Although not commonly traded in the UK, true Hypostomus plecostomus will tolerate low-end brackish conditions (to SG 1.005) and do so, for example, in Florida. Do note that in most parts of the world the "common plec" is NOT this species, despite what older aquarium books might suggest. The common plec of the modern hobby is some species of Pterygoplichthys, and I'm not aware of these catfish being particularly tolerant of brackish water.
Various species of bagrid catfish occur in brackish water: Mystus cavasius, Mystus oculatus, Mystus vittatus, Mystus gulio, Mystus wolffii, Hemibagrus microphthalmus, Sperata seenghala, etc., etc. Of these Mystus vittatus is the one most often seen and small enough (at around 20 cm) to be considered a peaceful if predatory aquarium species. It tolerates slightly brackish (around SG 1.003) conditions well. Mystus gulio is less frequently seen but has a much higher tolerance for salt water, easily taking half-strength seawater (SG 1.010). It is rather big (15-30 cm, depending on the site of origin, likely multiple species) and distinctly predatory, though otherwise a peaceful schooling catfish.
Ariids of various types are traded, at least in the UK, including Arius sagor and Sciades seemanni. These are big predators and not "scavengers" at all. They're peaceful though, and work great kept as groups alongside scats, monos, etc. Adult plotosids will do well in brackish water, but alas, they're hardly traded these days. In the past at least one species, Plotosus lineatus, was fairly common as a saltwater fish. For whatever reason, it has virtually disappeared from the hobby.
The larger banjo catfish such as Aspredo and Platystacus spp are brackish water inhabitants, but they are exceedingly rare in the hobby. They're big (around 30 cm) though eel-like, and need very specific conditions to thrive, namely a sandy substrate and plenty of worm-type foods.
Many of the Pangasiidae occur in brackish water, and at least one species, Pangasius krempfi, swims into the sea. Be that as it may, the sheer size of these catfish makes them dubious additions to the home aquarium. The same holds for the Ictaluridae and Siluridae, some species of which occur in slightly brackish water but get too large to be easily kept in an aquarium.
No catfish "cleans up" your tank. If you think that way, you're doing your catfish a disservice, as well as deluding yourself about how to maintain an aquarium. True, catfish will eat uneaten fish food, but if you're feeding your fish properly, there shouldn't be any uneaten food! So take catfish for what they are, and give them appropriate food items at night. A mix of catfish pellets, chopped seafood and algae wafers usually works well for most species.
As for loaches, the vast majority live in upland habitats well away from the sea. There are some species that tolerate brackish water though. The horseface loach is the most widely traded and does well around SG 1.003, perhaps slightly higher. There are persistent rumours of clown loaches being found in brackish and even saltwater habitats, but I've not read anything in the scientific press to confirm that, and for now would recommend against keeping them in a brackish water system.
Cheers, Neale
