Mammal fat, which congeals to a waxy substance when it's cold, is saturated fat. It's not digestible by cold-blooded critters such as fish. They do need lipids (oils/fats) in their diet, but they must be the unsaturated kinds.
That's why beef heart, with all the fat trimmed away, is okay. But beef flesh, intermarbled with delicious fat as it is, is not okay.
Of course many fish would eat a ham-and-swiss-on-rye if you dropped it in the tank.