How does it smell when you open it? Sometimes on a canister filter that has been regularly maintained, all you need to do is rinse the media in a bucket of tank water or treated water and put it back in service.
If it is stinky, I would rinse any crud from the filter, remove any chemical media, and then operate the filter and remaining media using a bucket of unscented regular bleach water for an hour then repeat with a bucket of Prime treated water for another hour then smell test to be sure all bleach odor is gone. 1 part bleach to 9 parts water should be the correct ratio.
The chemical media may or may not need to be discarded except for Seachem Purigen which can be regenerated according to directions.
Eheim Pimpette #270
Eheim Pro II 2028; Classics Series -2215, 2217, 2260, 2262; Pro 2229 Wet/Dry
Last edited by deeda; 04-02-2020 at 10:17 PM.
Reason: add info