WCMM are awesome - such beautiful little fish.
You might consider using a sponge filter to create some water movement - I do this in my 5.5 gallon Ghost Shrimp tank, just to keep things from getting stagnant, but it does generate a moderately decent flow.
You might also consider using a Marina Slim 10 just to create some flow as well - they are beautifully silent, self-priming, include a nice reticulated foam intake filter and are dirt-cheap (~$20 at Petsmart here in Canada). That's what's running on my Walstad 5.5 gallon WCMM tank. It just has a piece of foam in the filter chamber, but no filter material - and quite frankly, I'd be better off with nothing in there at all, since it's just for flow, basically.