I have virtually given up on these pumps. I have 4 of them. 1 nano - no probs. Bought a 750 works sporadicaly with no logical sequence of when it wants to function or not. Bought 2 larger evolution pumps from fellow club member - same thing. I gave vinegar bath and same issue. Was using the 750 the other day (it was working)....had it running for about 3 days. Go to take it out of one tank and place in another - unplug...wait a bit....take out....put in new tank...plug in....nothing. There is a slight vibration/hum indicating the pump wants to work....but nothing else. Unplug, inspect, propeller appears to spin freely...assemble again - doesn't want to spin. Try placing gaurd on different ways, same result. I am using these for straight ciculation - no wave makers and not turning on/off frequently. I have over $100 worth of pumps laying on the floor....ready for the hammer. Any advice for a frustrated reefer?