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 the Practical Fishkeeping Magazine titled;
Liver Damage and Red Meats,
"The routine of feeding beef heart and other red meats to Cichlids can ultimately give rise to health problems. Poultry meat is also suspect. Red meats, including lean meats such as beef heart, contain the wrong sorts of fats - these harden within the cold-blooded fish, leading to blockages and fatty deposits around the liver.
Also, the relative proportions of amino acids within the mammalian proteins are different to those required by fish. Hence, feeding red meats will cause the cichlid to excrete more nitrogenous (ammonia) wastes, thereby placing an extra burden on the biological filter."
HTH
