Ok this all started with a new fish that got sick and died? From the pic it looks bacterial. With the temp at 88 that’s only going to help the bacteria. Slowly lower the temp to 82. Try rid-ich plus. You don’t have a pleco in the same tank?
Frozen brime shrimp alone isn’t good for the discus. You need a mixed group of foods. Some use beef heart,black worms,flakes and granules. The more different varieties the better