Help with Discus fish?

My tank is 5 years old and I have three large anglefish, two small appisto cichlids, lots of tetras, and a two small gouramis. It is a 50 gallon freshwater tank. I have lost of driftwood, and it is heavily planted. I cannot keep Discus and I do not know why. One year ago I bought a pair of small turquoise Discus, but they both mysteriously died after 4 months. I recently bought another one (I decided to buy one and then introduce a second one a few weeks later if it lived because they are very expensive) and it also died. My ph is 6.4. Hardness is 20ppm (very soft.) Alkalinity is low, nitrates are 5ppm or less depending on when I make water changes, and I have never had any nitrite levels. I keep the temperature warm at 86 degrees, and I keep peat in my canister filter so the water is very soft, the driftwood also helps it stay like this. I feed them live and frozen bloodworms, flakes, pellets, and romaine lettuce. Please help.