What kind of biotope are you shooting for? Upland river or lowland river? Upland will be a faster flowing river with larger rocks and less plants. A lot of characiformes live in this water in the Amazon, tetras and minnows. If you do a lowland with less current and finer substrate, you can do just about anything south american ( angels like a very light current). If your not shooting for South american, try rainbowfish, bala sharks, red tail sharks, danios, the more streamline, long and thin the fish looks, the better it will do in a current.