Welcome to AA!
How long ago did you add the new influx of fish? If it was recent, and you haven't done-so check your ammonia and nitrIte levels. If these start to get elevated they can cause your pH readings to fluctuate. That may be the source, unless your water has always had a high pH reading like that. It's possible that an ammonia spike when you added the new fish is what caused your angels to perish.
Also, most fish (even angelfish) will do fine in higher pH levels (unless you're purposely trying to breed them). They prefer a stable environment, even if it's not in their "ideal" range. Stable is much more healthy for them.
Trying to control your pH is very difficult with chemicals. Unless you have a specific need to do so, I recommend leaving it alone.
So, my recommendation is to test your water for Ammonia and nitrIte. If either of these are elevated above 0 we'll need to work on that. Stop dosing pH down and just let your tank settle in to it's natural point, because it will stay there and be a much better environment for your fish this way.
