how are you going to shut off the pump that pumps water into the tank when the bucket goes empty? other wise, you are going to be running your pump dry.
also, it takes you 7min to drain your tank? what diameter tubing are you using? i have a 3/4" section of tubing, and can drain my entire 72gallon tank in less than 5min.
Put a pressure switch on the discharge side of the pump, as soon as the pressure drops it will cut out the motor.