On Saturday I wanted 8 ghost shrimp for my betta (he's in a 12 gallon). Mostly for extra cleaner, while also expecting the my betta trying to eat them. After one night, he ate out the eyeballs of 3 shrimp while the remaining 5 seemed to be attacked but not too damaging. Currently they're in a 2.5 gallon tank, no filter, gravel as substrate, 1 moss ball, 2 plastic plants, and a small castle for hiding. For food I put one algae wafer (in tank right now) and plan to put pieces of lettuce or cucumber after a couple of days.
My question is, do ghost shrimp need filter? I currently have 5 of them in the 2.5 gallon tank. If they do I'm planning on to get a certain sponge filter that's safe for shrimp and small enough to not cause too much current.