The following is a repost from a comment that I made here last year.
Fish aren't hard wired to assimilate the fatty acids found in beef, chicken, etc anymore than they are hard wired to assimilate large amounts of carbs. These excess lipids get stored in & around the organs, and eventually shorten the fishes lifespan. Can these foodstuffs offer amino acids, and solid growth, yes, no question about that, but that doesn't qualify them as being a good source of food for a fish.
Even a lot of the major discus keepers have moved away from beefheart over the past decade, for these exact reasons. It's a great food for breeders that simply want quick growth in their juvie fish (so they can take them to market quicker) but it is most certainly not an ideal long term diet. Lee Newman, Curator of Tropical Waters at the Vancouver Public Aquarium has spoken out against feeding beefheart many times, for the same reason as I do, it tends to lead to fatty degeneration of the liver.
Dr. Peter Burgess MSc, Ph.D.,of the Aquarium Advisory Service in England, is not only an experienced aquarium hobbyist, but also a scientist that specializes in the health & disease in fish. He has written over 300 articles and five books on fish health and is a visiting lecturer in Aquarium Sciences and Conservation at Plymouth University, where he works with the University of Plymouth training students in scientific research. Among his other positions, Dr. Burgess is a senior consultant to the Mars FishCare business and regularly runs fish health & husbandry courses for aquarists, fish scientists and vets. He's also a regular contributor & Fish Health consultant for the Practical Fishkeeping Magazine, as well as other magazines devoted to the fish keeping hobby.
Below is an excerpt from Practical Fishkeeping Magazine titled; Liver Damage and Red Meats, where Dr. Burgess spoke out against the use of mammal meat when feeding fish.
If someone can show me a long term feed trial performed by an accredited institution that involves feeding beefheart and/or chicken to catfish, with no negative health issues I'd be more than happy to change my position on the subject. The fact that some people feed this type of garbage to their fish, and the fish eat it, doesn't equate to there being "nothing wrong" with it.