The dwarf pygmy angels are sometimes difficult to keep because their natural diet consists of sponges. Since it's hard to provide the specific natural sponges they feed upon, their diet can be problematic if they don't accept prepared foods.
I had one years ago that would not eat anything I offered, I suspect harassed my prize maxima clam to death(probably for lack of food), and eventually disappeared in the tank.
I have a feeling they probably do best in older, larger systems. Good luck.