The SW will kill off all your FW bacteria. Canister filters with the typical media used for FW is not recomended for SW. You have to think totally differently about SW filtration. All of it is done with live rock & a skimmer, unless you plan on doing a lot of water changes (just like FW), which can get costly.
The expensive part of SW set up is the live rock (LFS can charge up to $9/lb) & a good skimmer. Best thjing to do os go to www.reefcentral.com & look for a local reef club near you. Some reefers ate tearing down or downsizing & selling LR as cheap as $1-3/lb.
lol, salt adds up. If you are needing to do large water changes to compensate for inadequate filtration it is going to quickly add up. And while the filter may be listed as FW/SW the bacteria is not the same.