Need inline timer shut-off relay switch for 12 volt application?

I have water pumps that fill and drain ballast tanks for a wakeboarding boat. The problem is that once the tank is full or emtpy one must remember to turn off the pump or else the pump will burn out (especially when draining since it runs dry). It takes about 10 minutes to fill or drain, so I want to have some... show more I have water pumps that fill and drain ballast tanks for a wakeboarding boat. The problem is that once the tank is full or emtpy one must remember to turn off the pump or else the pump will burn out (especially when draining since it runs dry). It takes about 10 minutes to fill or drain, so I want to have some sort of relay I can add to the electrical connection that will automatically cut the circuit after the delay. Are there any products out there that can do this? This is a 12 volt DC pump that is rated at 11.5amps.
Update: Also, the tanks are not rigid, so there is no possibility of a float switch. Filling is less of an issue because there is an overflow that drains above water level, so I can actually hear when they are full & can turn the switches off AND even if I didn't, the pumps would just keep pumping water which is not... show more Also, the tanks are not rigid, so there is no possibility of a float switch. Filling is less of an issue because there is an overflow that drains above water level, so I can actually hear when they are full & can turn the switches off AND even if I didn't, the pumps would just keep pumping water which is not really bad for them. However, draining is below water level, so I hear nothing, and when they run out of water, the pumps will burn up the impellers withing minutes.

The best solution would sense water flow & interrupt the circuit when the flow stops. I would love that if not too expensive, but I figure the timer relay would be easiest.
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