First off, you are beyond your stocking limit. The Regal and the Powder blue should be kept in large aquariums. Even if your filtration will completely handle the waste produced by these animals, they need "personal space." Without this space to feel safe, they are under constant stress. This will reduce their ability to resist disease in general.
To eliminate Ich from your tank, you need to have a fish free tank for ~6w. To cure your fish from ich, you will need to treat them for at least 30days in a hyposaline hospital tank (see my previous post in this thread). You could also use a copper treatment, but I prefer the hyposalinity method.
As for your angel specifically, It is a wonderful fish that has specific needs. If these needs are met, it will be much more resilient.
Good luck!
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