The canister housing itself requires rinsing every now and then to get that inevitable brownish gunk off of it. Otherwise, you're constantly recycling that stuff.
I probably do a complete rinse, housing and components) every other month or so. Depends on your bioload and your general tank habits.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~