Once in a while it would be fine. I wouldn't make it their primary food. The problem is carnivorous fish and rays in the wild eat whole fish, worms, and crustaceans. They eat the head, eyes, cartilage, skin, everything. By just feeding them salmon filet, you are just feeding them muscle tissue and altering the overall nutritional content of food found in their natural habitat. As ultimatejay pointed out this is more expensive than quality fish food. Good quality pellets are an excellent choice, if you can get your fish to eat them.
I feed my rays, gar, and knifefish silversides, earthworms (bought from a fishing store-never collect from your yard), and chopped whole, raw shrimp. My other fish (arowana, tinfoil barbs, various cats, silver dollars, and cichlids) eat primarily pellets.
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