Well, canister filters certainly have their place. And it's not just FW. However a sump/fuge is the way to go.
Really if it's set up properly, it wont cause you any issues. Your tank will be so much healthier than with the canister filter. So I have to highly recommend doing some research here on sumps and refugiums. There is a lot of information right here. Go to the search type in sump, refugium, fuge, and you can search by my name for a lot of info on them. But don't stop there, research macro algae as well as natural nitrate reducing sand beds ,NR DSB etc. Also live rock rubble, pod piles. Don't just search by my name though, look to many others for their input. Then when you think you really understand the concept, we can help you put one online, even build one if you like. Just remember though there's more to setting one up than just putting a smaller aquarium down there and pumping water through it.