Freshwater fish I‘d have if I had the money and dedication for that hobby:

Going only by fish that do in fact ever get sold in aquarium stores however seldomly:

Thalassophryne amazonica - a tiny freshwater predator convergently similar to a stargazer

Compsaraia samueli - a knifefish from river waters so deep and murky it has reduced eyes and enlarged jaws like a deep sea fish

Asterophysus - a catfish similarly inhabiting such dark river environments it looks like an abyssal animal and its stomach can stretch to fit prey twice its size

Cetopsis candiru - not actually the “candiru” that sucks blood but shares etymology with it, this dumpy looking fish is what amazon locals actually do worry about instead of piranha, will bore a hole straight into flesh and look like an idiot doing it

Chaca chaca - an “angler catfish” that imitates a rotten leaf and wiggles its little whiskers to attract prey that it vacuums up, named because it makes a noise like “chaca chaca” to attract a mate

A little more conventional looking but I’m also liking the pugnaciousness of Rhaphiodon here, though I think this is one of those fish people buy tiny and that’s not good because it’s meant to get several feet long and live in huge open areas.

Special mention to mudskippers which are actually brackish, and I kept one alive (or my mom did for me, at least) for almost a year as a kid which was a miracle because there was NO care information and we definitely were not keeping it correctly.

Special mention to “freshwater” flatfish because a lot of brackish species just get sold as such and there’s debate over identifying the ony species that actually prefers fresh