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I found a guy on Craigslist that was selling a big aquarium setup for $250, and noticed that he had a fluval fx5 included, so I got him to agree to sell just the filter for a very decent price (decent enough that if it doesn't work I could probably fix it or use it for parts).

Any words of advice on cleaning it before putting it into commission? I figured I could just use the bio media from my existing filter to avoid having to re-cycle the tank (assuming it's been dry and whatever bacteria might have been in the filter is dead). Should I try to clean the other media, or just replace it? Should I let it run a tub of dechlorinated water for a day or so?

All of their media should be replaced. I would replace the hoses too unless you are able to thoroughly clean it. I don't know much about the FX series, but you might want to clean out the pump. I would also run the filter (w/o media) a while... use a storage container as a test tank. If you have any doubts, you might want to disinfect it (a weak solution of bleach, 2 oz to a gallon of water) and rinse well and repeatedly and letting the object air dry overnight (preferably 24 hours). Also check for leaks and lube/replace the seals.