Originally Posted by sufunk
Do not feed freshwater fish to saltwater fish , it will make them sick. I believe it gives them like fatty liver disease which eventually keeps them from being able to digest any food. Not sure about that but you definitely shouldnt feed freshwater fish.
I think everyone pretty much covered this topic well. The simple answer is don't do it.
Because you said you were not sure, I wanted to reinforce that you were indeed correct. Freshwater FISH (not inverts like ghost shrimp) cause a disease known as fatty liver disease because the fat content in freshwater fish is 5 times higher than in the natural diet of saltwater fish. After time, like you said, they will not be able to digest foods and absorb nutrients into their body.
I also wanted to add that I do not even condone or recommend an occasioanal feeder fish as a "treat". After a while, your fish will grow accustom to the fact that you give him "occassional treats" and he may stop eating the healthy food. I have seen this first hand, so this is just another reason not to do it. I am not saying they will always stop eating prepared foods, but the possibility is there.