If you really really want to keep daphnia only, you don't need a filter at all. Just a very very small powerhead with a larger sponge on the inlet (so the critters don't get sucked in and ground up) and on the outlet either a tiny spraybar that provides minimal water movement throughout the tank, or another sponge should work... If you are not much of a DIY'er, you could get the smallest available internal filter, and reduce water movement with sponges on the outlet.
Another way to avoid sucking in the little hoppers is a DIY filter. I described that in my cheap 43 gal tank setup, I am going to update the link to that one very soon. Basically you take a foam pad that is as tall as your tank and fix it with some plastic thingies in one back corner. The compartment then can house a tiny powerhead which pumps water back into the tank, as well as heater and CO2 if used. The water flows very slowly from the tank into that corner compartment, and nothing gets shredded...