I don't think you need one(filter), I have seen some very nice natural(planted tanks with no filter)tanks with shrimp and fish for that matter. You do need a tank with actively growing plants, to oxygenate. Natural tanks, they are a little tricky at first but once you get the hang of it, well worth it. Research the Walstad method of planting tanks..... When in doubt throw in a sponge filter for your shrimp.