Certain fish adapt to what seems to be offered as a food source. It's their instinct to graze all day long, not just when fed. The same way mandarins may finally realize that cyclopeeze is as edible as pods, likewise the court jester goby---they adapt. If we routinely expect these fish, with a very narrow range of acceptable food, to adapt to a total change in diet, it's not too surprising that angels, which naturally have a fairly varied diet, can adapt to what we grow in our tanks. The fact it's rare and more expensive than they are does not cross their radar.