Nothing really, I don't have anything that could slip into the filters other than shrimp lol but I have a piece of foam over the filter so they couldn't anyway. Doesn't get very exciting for me other than fish poop.
My tanks (so far
5.5 gallon Planted Dwarf Puffer tank (2 Dwarf Puffers and a lot of RCS!)
16 Gallon Planted
(GBR pair, 3 amano shrimp and a whole bunch of snails!)