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They sell a product that takes the output of a pump and distributes it to 4 or even 8 outputs. It has a drum that slowly rotates and creates alternating flow in the tank. I did know the site it was something like oceansmotion.com but I can't seem to get there.
They are proud of their stuff. I have wonder why someone has not tried to rig something with a little servo motor and a piece of flexible pvc. Just set it up to osculate back and forth. should be less than $20 to make and does the same thing. If I get some time soon I just my be that someone.