First off, Fish Only tanks are a bit harder to run. Without much BB in the tank, it's hard for it to digest. You need to have live rock for corals also. So in other words, you really want a FOWLR
system. For a 40b IMO
a sump would be better. I have one word for the canister: NITRATES! You need to clean them well weekly to keep them down. An AC130 would probably work well too. I've never had a 40b personally, but I'm planning on one, and I'm gonna do a sump.