Don't get me wrong, they're great fish. But if you put a 4" angler in your tank, you aren't going to be able to put any fish smaller than something like 7" with it,and I can't think of any 7" fish (except eels, which are too thin for angler housing) that would also fit in a 45g. Not happily, not with the amount of live rock you need in a saltwater tank, especially one with a messy thing like an angler. Anglers are really inactive, though, so I suppose you could put a mesh divider in the tank and keep something small on the side away from the angler. They don't exactly need a lot of space to move.