I'm sorry, but feeding mammal meat of any kind to a finfish is No Bueno in my books. You asked about papers/studies, Amy, so here's a link to a more recent scientific paper that I think every discus owner/breeder should read.
http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1679-62252008000400008
"This species feeds predominantly on algal periphyton, fine organic detritus, plant matter, and small aquatic invertebrates."
"The alimentary canal of Symphysodon is characterized by a poorly defined stomach and an elongate intestine, some 300 mm long and 3 mm wide (in a 180 mm SL specimen). This intestinal morphology is typical of a cichlid with a dominantly vegetarian, detritivorous, or omnivorous diet."
Heiko Bleher wrote the following…………
What do Discus eat?
I have examined hundreds of specimens during many years and stomach and gut contents among wild Discus indicate an order of precedence: detritus, then plant material (flowers, fruits, seeds, leaves), algae and micro-algae, aquatic invertebrates and terrestrial and arboreal arthropods. The Amazon has adapted to nature for fishes during millennia of evolution. Plants of the tropical rainforest have little water and cannot flourish during the dry season so cannot waste energy. The same happens to most freshwater fishes.
During the dry period, with a much reduced water level and hardly any food source — except for predators — many fish starve or feed on the little available, usually detritus.
Discus and many other fishes eat what they can get, but have to be constantly aware of carnivorous predators. During the six to nine months of floods, almost all trees and bushes, flower and have fruits and seeds — which is the main nutrition of roughly 75% of all Amazonian fishes.
The adults, and babies which grow to adults in that period, can then fill their stomachs and guts.
The carnivorous predators starve as they cannot find their prey in the huge water masses.
How much nutrition?
I have found the following percentage of nutrition in each one of the three species:
Symphysodon discus during low water: 55% detritus; 15% plant material; 12% algae and micro-algae; 10% aquatic invertebrates; 8% terrestrial and arboreal arthropods. During high water: 28% detritus; 52% plant material; 5% algae and micro-algae; 3% aquatic invertebrates; 12% terrestrial and arboreal arthropods.
Symphysodon aequifasciatus low water: 52% detritus; 18% plant material; 15% algae and micro-algae; 13% aquatic invertebrates; 2% terrestrial and arboreal arthropods. High water: 8% detritus; 62% plant material; 8% algae and micro-algae; 5% aquatic invertebrates; 17% terrestrial and arboreal arthropods.
Symphysodon haraldi low water: 39% detritus; 9% plant material; 25% algae and micro-algae; 22% aquatic invertebrates; 5% terrestrial and arboreal arthropods. High water: 6% detritus; 44% Plant material; 12% algae and micro-algae; 16% aquatic invertebrates; 22% terrestrial and aboreal arthropods.
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Not exactly a species of fish geared towards eating high protein foods such as mammal meat, and the fact that many discus owners still feed their fish foods such as beef heart doesn't equate to it being an ideal form of nutrition. According to the most recent science available, I would think far from it.
Several years ago Chong et al ran a 3 month feed trial on juvenile discus (fish approx. 4.5 grams in weight) and concluded that a diet consisting of 45-50% protein, and 8% fat was ideal for optimum growth for juveniles of this species. I have no argument with those stats, and again the same could be said for thousands of other ornamental species of fish, but somehow this data has been used by certain segments of Discus keepers to support their use of a high protein diet such as beef heart. I have never understood the logic in that.
Chong et al used fish meal as the source of protein (along with casein & gelatine as binding agents) in their study, not beefheart.
Even a lot of the old school discus keepers have moved away from foods such as beef heart over the past decade, for these exact reasons. It's a great food for breeders that simply want quick growth in their juvenile fish (so they can take them to market quicker) but in my opinion it is most certainly not an ideal long term diet due to the potential of fatty degeneration of the liver.
Jack Wattley has recently stated that a good staple pellet or flake food is more ideal for optimum health. In the Dec 2006 edition of Tropical Fish Hobbyist magazine, Jack stated:
"I've moved in a new direction regarding the feeding of discus, and after many tests feel that a top quality flake or pellet food formulated especially for discus is perhaps the best direction to take.”
The late Dr. Schmidt-Focke was one of the first to realize health problems when feeding foods such as beef heart, and quit feeding his discus beef heart in favor of a seafood based diet. Dieter Untergasser has also demonstrated the harm beefheart can have on discus and other long lived cichlids. And there are studies that have taken place that demonstrated that when too much protein is fed to a juvenile discus it can have the opposite effect, as it requires energy to excrete the excess amino acids (protein), which is energy that could have been used for growth.
It is my opinion that the goal should be to closely match the amino acids, fatty acids, vitamins etc. as the fish would receive if eating in the wild. With today's commercial foods this is much easier to do than 30-40 yrs. ago, and with a nutrient dense commercial food a fish will generally always be consuming more nutrients on a daily basis than they typically would in the wild. This equates to steady healthy even growth in a fish. Perhaps one won't see the massive gains as they would when feeding mammal meat, but I personally don't see slower more even growth a negative.
