My first

[warthog5]

the pump impellar will usually correct itself if you have them (the fins) in backwards.

While Miles is right, I hate to see the impeller vane's installed wrong and have to flip over to go the correct direction.

This could very well be part of your problem with the impeller self distructing.

Figure out which way the motor turn's and then the correct rotation of the pump.

Install the impeller with the vane's so they will NOT flip over, also lube the impeller with some white lithum grease when you install it.