Can smell when canister (s) have been neglected too long and waste collecting there is going/gone rotten.(stinks) Properly maintained, the media inside smell's like wet dirt = good. I do not run pre filter's on my community tank,(Turquoise rainbows,celebs rainbows,swordtails) so a few cherry shrimps that live therein, are always found inside the filters when I drain the filter's into bucket for back flushing the eheim 2217's. They seem to be alive and well living in the canister's, but as mentioned,,I clean the canister's every four week's. Used HOB aquaclear 110's for year's and media in these was exposed to far more O2 than that found inside sealed canister's.(never rotten smell) I was one of those who believed that a canister only needed cleaned every few month's, but my fishes seemed to suggest that this was not true. I keep,breed,trade, longfinned albino Bristlenose in another tank, and this tank needs filter maint nearly every two week's 55 gal with Rena XP3 and aquaclear 802 power head for added water movement near the surface. Tank holds 1 old Royal pleco,two female and one male Bristlenose,six panda corydoras,and two or three dozen fancy guppies along with gobs of cherry shrimps.(pre filter on intake of this tank) All tanks get 50 % water change each week. Maybe twice weekly when baby bristlenose swarms are produced. Perhaps I keep more fishes than other's so waste is more than other's might see. Have had nothing but good result's with afore mentioned maint.