Lesley
I understand your point of not putting an 18 inch fish in a 24" tank but if you get a JUVI angel (3-4") , it would take years for that angel to that big. Also , 18" size is attained by fish that stay in the Ocean , not in a fish tank . If and when it gets to that point you can either upgrade if give it to someone with the right size tank. If everyone followed the 250-300 rule, then the majority of people on this site wouldnt have these angels, which many do .