There are fresh water ones Gymnothorax polyuranodon. It is native to Australia and from what I gather very rare to get ones that are actaully the the true fresh water variant e. I heard about 5 years back of a pair going for $5k. The others that normally float around are regarded as freshwater the same as bulls sharks. They do best in brackish water long term, although from what I have heard those have a poor survival rate full stop as usually they are from a marine environment but of the same family.
Btw I am not expert this is just from what I have heard. Hardcore flush water forums may have some boffins.