I would add that, based on my experience, you can also use airstones to produce sufficient movement. Our 10-gallon has been without a filter for several months now. The only current in the tank is the result of one small airstone. It isn't much. But the tank stays remarkably clear with two albino cats, a clown loach, a bamboo shrimp, an apple snail, and twelve billion trumpet snails. I suspect this clarity has something to do with the duckweed also.If you are familiar/w those air driven sponge filters...there is nothing that says you couldn't just use the up-draft tube from one of those for water movement. Also for more of it in the on-line stores they sell replacement tubes for undergravel filters and those are 1" in diameter. And those same companies also sell the spigots with the righ hand angle at the top
for the tube.
Since my tank has a filter it's not in here. But I built in the filter which works on the same principle using air instead of a pump motor.
I did this specifically so that baby fish/shrimp and daphnia can go through without being hurt. In the first picture the water comes in on the lower left, up and then down through the filter media and back up through a tube that lets out where the bubbles are in the second picture. It has no screen/pads that can trap those mentioned critters.
But you can just use the pick up tube of an undergravel filter to move water without hurting anything.