Fx5 and mag pump for aquarium water change help?

Hi I have a fx5 but not sure how the water change works I know that it has something to do with the purge valve. On another note I heard of the mag pump is a great way to remove water from tank to the sink or tub, but not sure how the mag pump works ?? do I put the pump in the tank and stick tubing to the mag pump while submerge in water and the other end of the tubing to the sink? I have a 175 gallon what mag pump do you recommend ?