i'm sorry but i have to disagree with you there.
angelfish are pretty hardy fish...to kill one by stress would require a willing act on the person to get the fish so stressed. most angelfish sold in the hobby are many many generations inbred in farms and petshops and they are able to acclimatize to most water conditions readily.
more importantly, STRESS is not a disease, therefore it cannot be passed down. if the "stress" is caused by a sudden dramatic ph change, it can cause columnaris. if it's caused by temperature change, it can bring out ich. both of those are contagious, and columnaris is lethal if not treated well.
what it sounds that you have is what is called the angelfish virus. and i say that because i actually am dealing with it at the moment. the fish will seem fine for a while, usually up to a 1-2 weeks, and then suddenly die. while all your other fish are doing great.
there is no way to treat the virus, all you can do is help the fish's immune system by keeping the water clean and not causing any unnecessary stress. some fish can survive through it. if all the angelfish die, there is still a 3-week infectious period where you risk infecting any other angel you put in. if the angelfish survives, then it will become a carrier for up to 6 months before the virus exits its system...
in my case: i had two big angels, one from a petshop, one from breeder A. i introduced two more fish from breeder A. right away the older angel from breeder A (platinum) began showing symptoms of hemorrhagic septicemia, which came on as a secondary bacterial infection. i didnt know about the angelfish virus at the time, and tried to medicate with all things i knew about to no success. the fish didnt die, but didnt get better.
then i was told about the virus by breeder B and decided to let things run their course. i kept the water clean and the fish began to make a recovery. then my older angel, from the petshop, within 5.5 days began showing symptoms of the virus and died, while the "original" platinum was still recovering. the platinum angelfish has now made a complete recovery, and the original fish that are believed to have introduced the virus are still here.
so i decided to test the theory. if it was the angelfish virus, any new fish introduced would die and show the same symptoms. so breeder B gave me an angel to sacrifice. for 6 days it was doing great, and then a period of 7 hours it began showing symptoms and died...so as you can see, if those 7 hours were while you were sleeping, you would never have known why the fish died!
so think about the pattern of deaths that you've experienced and see it if matches with the description above. hopefully it wont!
