How long does it take an angelfish to grow to adult size? I have an angel that I got about 2 months ago. He was originally about the size of a quarter, and now he's about half again as big. Lately he doesn't seem to have grown much, though, so I was just curious if something is wrong or if they are slow growers. He's in a 55-gallon tank, so size should not be hindering him. He seems to think he's one of the black skirt tetras, though, and actually schools with them. I was worried that there would be squabbling between the two species since both have long, flowing fins, but they seem to get along just fine.