Reef tanks are mini ecosystems. Where the animals all play a roll. Where would these fresh water fish fit into this environment. Their offspring will not survive in saltwater, as far as I know,.And as to the fry being food for other inhabitants, I don't believe that they would have the right protiens for saltwater animals. Mainly do to the lack of omega 3's. Then there is the question as to long term consequences. Saltwater and freshwater animals have immunity's to their respective environments, would introducing freshwater fish into saltwater open the other inhabitants to deseases and parasites, viruses and bacterias that they have no way to fight off.
I can understand scientific studies doing these things, but as to reefers and pet stores doing it , I think we should let nature tell us what environment they should be in.
These are just my opinion, for what it is worth.
RIch