Angels can be sensitive fish. Angels like discus, I would not buy at the LFS. They just don't have quality. You probably got poor quality fish, that were already sick.
If you buy angels again, get them from a good breeder, just have them Fed Ex'd to your door. You'll be miles ahead just by doing that.
Angels need warmer water, try above 80f or so. They dont like it too much cooler, and it also effects their immune system. They are sensitive to nitrates. 40 ppm of nitrate is too high. Try for 5-10ppm. Your water changes depend on your chemistry, and what's in your tank. Once a week might be fine. If your nitrates are more than 5-10 ppm increase your water changes. Also do big water changes, this 10-20% stuff is not worth the bother. It's like getting a decafe latte, why bother? At least 50% is needed. The frequency will be determined by your chemistry. Also throw out those test strips, get the API master kit. The strips are not accurate enough. When you do the nitrate tests, shake the bottles very vigorously for over 3 mins. Otherwise the reagents precipitate out of solution. You have to shake the reagents very vigorously for over 3 mins so it's suspended back into the liquid. If you don't shake enough, you will get false low numbers, and your fish are exposed to higher nitrates than you know.