LisaLQ is absolutely correct as far as the commonly traded species go, but it is actually a bit more complicated. Hypostomus plecostomus, for example, is established in brackish waters in Florida, and there are even a few species that naturally inhabit brackish waters in the Amazon, such as Hypostomus ventromaculatus and H. watwata.
http/www.fishforums.net/index.php?s=&...t&p=1103634
Unfortunately, your chances of seeing these species is between almost zero and absolute zero. Moreover, some of these species likely migrate in and out of the estuary following the freshwater water and avoiding the saltier water, so may only be exposed to brackish water for short periods (though Hypostomus plecostomus at least does appear to be a permanent resident of brackish water ditches in Florida, where it's actually quite common).
I have no idea what the salt tolerance is of the commonly traded species is, but it's likely to be low. I've seen a royal plec kept in a brackish tank at my local Maidenhead Aquatics, and it's in a tank with a Celebes halfbeak as well as scats and monos, and I'd have to assume that the SG is about 1.002-1.003. The mere presence of salt doesn't kill catfish (that's a myth --
catfish are very common in brackish water, and there are dozens of marine species). But for all practical purposes, aquarists, unless they know exactly what they are doing and precisely which species they're keeping, are safest leaving plecs out of brackish water tanks.
For algae control, mollies and Florida flags will do perfectly well, as will Amano shrimp. Combine these with fast growing, salt-tolerant plants like Vallisneria, and you should be algae-free.
Cheers,
Neale
Nope, definitely no salt.