A fish does not have to be sick or dead to get stuck on a filter intake! Small fish, or clutzy fish with long fins can get trapped, too! And being stuck just for a few seconds can be enough to tear a long fin. Fancy goldfish have this problem, big time.
I cannot find foam filter material in my area easily, so I get creative with plastic mesh, the stuff crafters use. It comes in colors, but I mostly use black and dark green. I recently made a cylinder, with a round piece at the top and bottom, cut to fit my filter pipe at one end. I used nylon fishing line to tie fake plants to it, and it "disappeared" visually. Zero chance now of a fish getting too close, but the filter can still work with no loss of suction.
I also use the mesh to make filter boxes for my outdoor ponds.