Feeder crabs.

[matongflush]

Does anyone know whether RTCs will contract sicknesses of any sort because of being fed marine fish or crabs?

[sidguppy]

Good question!

I feed my freshwater-fish often with marine shrimps, but those are usually cooked.

cook your crabs and any germ or parasite will be dead.
feeding cooked crustaceans is perfectly safe, if you feed sparingly.
overfeeding will fatten your fish and pollute your water