Beef heart seams to be a staple for large carnivorous fish, and i dont think its a great idea to feed it.
Beefheart obviously comes from a warm blooded animal, and any fats from a warm blooded animal have a high melting piont, which means the fat is mostly going to be stored in the liver.
On the other side of the coin, cold blooded animals have slow melting fats. That fat is used and stored as energy, and can be burned off.
Even though beef heart has a very low fat content, prolonged feeding of it will not give good results. Once in a while will be fine though.
Stick to meats from cold blooded animals like fish, shrimp, ect...
Use a high quality pellet as a staple, and use cold blooded meat to complete the diet.