Not to correct the mighty Corax (seriously), who's taught me many things, but I believe that a frogfish could be kept in a 20. It's somewhat similar to pac man frogs. They get huge but are generally inactive, so space isn't as much of an issue. However, they do need good water and feeding can be hard. You need to get a pair of tongs (recommended at least) and you have to target feed, as the fish in the wild wait for food to come to them, so any pieces not eaten immediatly will just rot. As stated above, they aren't active, so don't expect alot for personality. I was thinking about getting one, but decided not to, at least for my first marine tank. Also, they cost alot of $$$.