You can use a canister filter for SW
. It will just require periodic cleanings. I used Fluval canisters before, and they do a good job for the most part. I cleaned them out about once a month.
What kind of rock substrate do you have in the canister? I have heard of some people loading the canister up with reef rubble, and removing all the other contents for a more natural filtration. This way, there is nothing to replace. You will just need to clean the canister during regular maintenance. Just make sure when you do clean it, use SW
as Fenix pointed out. HTH