Filter Media
Filter media is where the biology of your tank actually lives, and it is the part people most often throw away by accident.
The three jobs, in order
Mechanical media — sponges, floss, pads — traps particles. It clogs, and it is the layer you rinse most often. Biological media — ceramic rings, porous blocks, mature sponge — houses the bacteria that process ammonia. This is the one that matters and the one that should almost never be replaced. Chemical media — carbon, resins, phosphate removers — strips specific things out of the water. It is the only one most tanks can do without.
Do not replace the cartridge
Disposable all-in-one cartridges are sold on a replace-monthly schedule, and following it throws away your bacterial colony every month. That is why some tanks never seem to stabilise. Rinse the cartridge in old tank water and put it back. Replace it only when it is physically falling apart, and when you do, run the new one alongside the old for a few weeks so the colony transfers.
Rinse in tank water, never the tap
San Diego water is chloraminated, and chloramine kills the bacteria you are trying to protect. Siphon a bucket during your water change and rinse media in that. Squeeze sponges rather than scrubbing them — you want the gunk out, not the colony.
On carbon
Carbon is useful for pulling out medication after a course of treatment, or clearing tannins if you do not want the tea-coloured water driftwood produces. It is not needed continuously, it exhausts in a few weeks, and left in during treatment it will strip the medication straight back out — which is the most common reason a medication appears not to work.
See also filters and water treatment.