Oh sorry to hear about your fish. It might be hard to tell exactly what caused the fish to die. Sometimes they just die for no apparent reason. Your ammonia and nitrite are zero which is great, but your nitrate is starting to become a bit much for angels. They are not as sensitive as discus but they like low nitrates, try to drop the nitrates to like 5-10ppm. Even though your ammonia and nitrite are zero which is great, but to me it seems like that's a lot of fish for the tank. Overcrowding will stress angels.
Angel fish need a high protein diet that's varied. The blood worms are fine as a treat, but there's not enough protein in it. Feed the angels 2-3 times a day, and maybe one of those feeds could be the blood worms, but the other two feeds make it high protein flake. Otherwise they will be malnourished. Angels have a big body so they need the protein. Also with a temp of 80f it revs up their metabolism.