Oops lol.I must have been feeding in about 40% of their diet prima/flakes...I'd like to know where you get they only live for 4 years? Beef heart varies greatly in quality, according to most it is best to make your own (again this breeder made his own lol), and there has been no scientific proof that it is bad for them as far as I am aware, and it is continually used by breeders to give discus optimum protein for growth...Although I definitely wouldn't use it exclusively...Just not natural is it.
Definitely, they can be very susceptable to some diseases, e.g HITH, nd this disease on appearing killed them overnight (2 of the 6 of them).
I'm going to bed- so tired.
I was told by a discus breeder who fed primarily beef heart (this was home made) that his Discus lived 3-4 years on average. There is proof linking a high protien/fat diet to a lesser life span in Discus, I am not sure if I have link book marked (I do have a really good discus diet article) either way I am not using my own computer as I forgot it (on vacation

see if I can do some digging later.) The lowered life span is primarily due to health problems from discus being over weight. Yes a lot of breeders use beefheart, a fad ushered in by Jack Wattley due to it being a good and cheap source of growth. Discus breeders will normally fed heavily on beefheart to grow them out quickly and cheapily for sale, but this is only until they reach the 4 or 4.5 inch range and then I hear some breeders slowly ween them onto other foods (although a lot of times beefheart is still fed.) Using it for a short period of time as juvyniles should be fine, it isn't such a big deal for me, as they need the extra protien and fat in order to grow properly, but I would be angry purchasing adult Discus that have been fed primarily beefheart for their whole lives, even reputable breeders can be greedy

The link I had, showed a study about Discus eating a lot of vegatation in the wild (it even showed a break down of a single wild discus(es?) stomach content) and inside the stomach the discus had a very large amount of vegatation and not a lot of protien/fat, the shape and size of the digestive system and intestines also pointed to a fish that is mainly omnivorous. This shows Discus should be fed a decent amount of vegatation (a part of the diet commonly missed.) You also have to remember it takes 2 years for a Discus in the wild to get to adult size, discus on a high protien diet can be grown to adult size in 7-8 months (again the main reason for breeders using it.) This alone proves that feeding such high amounts of protien/fat is unhealthy and unnatural and IMO shouldn't be used any longer then getting them to the 3.5-4.5" range (but ask some knowledgable disucs keepers and the will say NO beefheart ever lol.) But team that up with that fact that In Discus fat is stored mainly in the liver, body, head and kidneys (similar to humans) and health problems are normally similar to the ones found in over weight humans, and that should be solid proof Discus and high amounts of fat/protien should be avoided.