I would not recommend beef heart to fish. It's just not good for them. If you do decide to give it to them ,give them a small amount once in a blue moon.
beef heart is far from a natural food source ...Iv read numerous times that fish cannot properly digest mammal meat, it leads to fat unhealthy fish (though bigger yes)
While certainly not a natural food source for Bichirs, I have found it a good source for my fish. I use beef heart as part of a rotation of foods and feed one time per week. Here is some shots of how I prep it.