turnning pumps off and on

[Delphinus]

I looked into this myself a few years ago or particularly using a MOV or two to switch direction every few hours or minutes but the pump stays on. Then basically the Ocean Motions came out with the same idea although I don't know if the speed is easily controlled.

Switching pumps would work also except you need two. On and off on a closed loop is not so bad in and of itself if you're looking at a few hours per cycle but I would think it's still harder on the pump long term than just leaving it on.

You're basically talking about doing a gyre flow, it's a good idea in principle and there seem to be a few ways to achieve a good result. Coordinated powerheads on a controllable wavemaker can work towards this as well.