I've been tossing around the idea of a gravity/tidal surge grow out system. I've never seen anything like this and think working out the logistics would be a fun challenge. And if it's possible it would cut down on electricity by only needing a small return pump.
The tanks would be 32X24X12.5(40gal.'s). Not sure the size of the sump or how I would have it configured. The drains I think would be bell siphons and drain from the 1st tank to the 2nd tank which almost immediately would fill and drain the 2nd tank to the 3rd tank which would be tied to the sump at the ground level. I might need to put a baffle in the 3rd tank closest to the sump to prevent the water level dropping in the 3rd tank while the sump's pump is filling the 1st tank. Not sure if the baffle wasn't there, if there would be a chance of the 3rd tank draining to low. Or if when the 2nd tank dumped if it might over flow the 3rd tank before the water would be able to equalize into the sump. Or if the volume of water would allow for enough water to drain from both and not drop to low in the 3rd tank making the baffle in the 3rd tank not necessary. Might be able to solve that with larger bulk heads and PVC pipe between tank 3 and the sump. Say 2, 2" bulkheads on both the 3rd tank and the sump? Or instead of the return pump being outside the sump, put it in the sump which if it was a mag drive would create heat which would heat the water. And instead of the return line coming up over the tanks rim it could go up to a ball valve up threw the bottom of the 1st tank. Anyways I threw this together it's a very crude drawing in paint to show a visual.