Originally Posted by Greenstreet.1
I think the reason for him saying so is because the minimum recommended size is a 70gal.
I would definitely think twice before putting a flame angel in a 40 gallon. They get big. I recently saw a fully grown coral beauty which was about 4-4 1/4 inches long. that fish was big. I began to wonder how people keep them in 24, 28 and 29 gallon nano cubes.
However, a cherub angel could comfortably live in a smaller tank and having kept one in a 30 gallon I wouldn't say that having a cherub in a 40 gallon is pushing it. As they get territorial it's important to add them last.