USMC is on the right track. Typically it is not recommended for a salt tank to run a canister filter. However many people have forgotten about fish only tanks. Nitrates, trites, ammo, are all very bad in a reef tank. A canister is known to produce those items.
I wouldn't say you should toss it out.
Canister filters are good in fish only tanks as the fish are able to withstand a higher elevation in parameters more than corals and inverts. One thing to keep in mind is that it should be used more as a mechanical filter than as a biological filter. As in you should plan to clean it once every other or once a week.
Carbon is ok and recommended to run in salt tanks. But it has it's limits. You are far better off running it in the fish only set up. It will not do much harm. Plan to run it more like every other cleaning. That way it won't suck up as much of the beneficial suplements of the salt mix. It is only recomended for use in reef tanks when something bad is happening. If you need to quickly isolate a problem, say a dead fish, run it for a couple of days. The reason you would not want it running continuously is that it will absorb to much of the suplements necessary for a thriving reef.