A lot of people on here try to claim that fish can't digest mammalian protein. But yet they never have any proof of there claims. I can tell you for a fact that my group of 7 discus grew from 2 inches to all 6-7 inches in about 1.5 years on a daily regiment of beefheart and turkey heart. They got a small amount of flakes and bloodworms, but about 70% of there diet was beefheart or turkey heart. That's a lot of growth for feeding food that cannot be digested. I also feed my bigger cichlids beefheart cubes and turkey/chicken liver from time to time as a treat. I have been experimenting with cubed beefheart and cichlid delight (has a good amount of turkey heart) with my geo's as well and have been getting some good growth.
The key is to feed foods that do not contain very much fat. Animal fat is indeed bad for your fish. Liver, gizzards, hearts and such have little to no fat and are a excellent protein source. Decades of dicus breeders have pretty much proven this. If you want to feed these foods and not use the precubed stuff you can by at pet stores you can get it at almost any meat counter. You can chop it up real fine and use a mix as suggested above with spirila, shrimp, beefheart, flakes and such. Put it in a zip lock bag and flatten it out and stick it in the freezer. Break off a piece to feed. You can also just pitch in the pieces of liver, gizzard, ETC that is the correct size for the fish you are feeding. Just wash it really well first to get any blood out of it.
These foods do tend to dirty up your tanks a little quicker, so either up your water changes, feed less often, or give it only once or twice a week.