diseases pretty much never have issues due to the completely different osmotic pressures. Parasites will typically explode/implode and bacteria's will not live either due to the extremely different pH values. That said, the FW
fish will have little nutritional value for the eel and can often cause intestinal issues.
Alternatives would be shrimp, clams, mussles, squid and the like. You can even convert black mollies over to salt and breed them as a food source. After a few generations, they should offer good food value.