I found this when someone asked if they could use beef heart to encourage growth in their goldfish. I don't know if it is correct or not...
It's safe and will provide some nutritional value, but to be honest it's not really the best source of protein for fish. Your fish will be better off if you avoid meats from warmblooded animals such as beef, chicken, pork etc and feed from coldblooded sources such as insects and other fish. Things such as shrimp, clams, etc also fall in that second group.
It's a fine line and a small detail, but the amino acid balance is far better in the coldblooded sources of protein for your fish.
Many Discus breeders have and continue to use it as part of their fish's diet and it works, but only as part of a very well balanced diet and only when superior products are not available.
To feed beef heart you will need to get the hearts and remove all of the tough membranes, valves and other tissues like arteries. Then chop it very fine or place it in a food processor and basically turn it into course hamburger meat. you can then feed small bite sized pieces of this tot he fish. I can be frozen in a fairly thin layer to preserve it and help you with portion control.