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Is carbon (chemical media) and ceramic rings (biological media) really needed in a filter? What do they do?

Biomedia along with mechanical media (like sponges) is a must. Carbon can be useful but is not absolutely necessary.

Biomedia provides surface area for beneficial bacteria to grow, and it's the beneficial bacteria that break down ammonia into nitrate and then nitrate (which is removed from the tank with water changes).

For me, carbon becomes more important in smaller tanks to help control any odor.

If you're using a canister filter and choose not to use carbon, fill the empty space with biomedia. If you're using an interal filter, there mostly likely isn't room for any significant amount of biomedia.

Do you have to replace the carbon or ceramic rings ever? Thanks for your help!

The only real uses for carbon are as Marisa said to remove chemicals like medications or odor causing chemicals formed by the BB. It is not necessary unless you are trying to remove a chemical. The ceramic rings are media, not carbon, and yes, eventually they will break down and need to be replaced but it takes a loooooong time.

The ceramic rings, probably not for a very long time. When you clean the filter, rinse them and keep them in some of the tank water (I keep a bucket for this purpose) until you're ready to assemble the filter. This will help preserve the beneficial bacteria, which, if destroyed, need time to grow again. BTW, you also want to rinse any sponges, etc. in tank water when you clean them (and keep them in the tank water until they're put back in the filter).

Carbon loses its effectiveness over time has to be replaced, usually monthly, but it should tell you on the package how long it can be used.

Your welcome.