One type keeps water free of debris so that the pump won't clog. The other removes chemicals that harm fish. To sustain fish, you'll need both types or a product that puts both in one package.Mechanical
Traps debris before it reaches the pump. Clean weekly in spring and fall, every other week in summer, monthly in winter. A filter in a waterfall or a surface skimmer will be easier to reach than one on a pump at the bottom.
A must for ponds with fish. Bacteria living on a porous medium digest toxic nitrites and ammonia. Wait six to eight weeks for them to become established, or buy a starter colony. No cleaning needed.