[kaptken]

Well GB, there certainly is no free lunch when energy recovery is concerned, but there is subsidized. We could recoup some of the energy expended in raising water from sump to tank and falling back again. It seems to me we did a thread like this a couple years back on the large reef tank forum?

Regular, sealed tight clearance turbines would be a very bad choice for many reasons. First they need a lot of flow with a very large head of water and no bypass. that would be bad when the impeller shaft gunks up with calcium and holds up the impeller. back up and overflow the tank.

My best suggestion would be to build a 3 foot diameter acrylic bucket wheel, like an old colonial overflow gris mill water wheel, supplied by the tank overflow return to the sump. enclose it in a gasketed acrylic box with lots of room for the water to overflow should its shaft get impacted with calcite. the large diameter wheel would provide low speed torque to drive an external pulley and belt system geared to turn a DC generator. and preclude tank overflows. The DC power could be conditioned with an inverter to run a pump or powerhead. or heater. recapturing some of the spent watts by gravity.

But a much simpler app would be to install a few 1 1/2 volt solar cells in the light hood to collect reflected light, and run the hood DC computer type exhaust fans. Lights on... fans on.. no need for extra power cord, transformer, and timer.