[Ryland]

There are a lot of magnetic pumps out there that have the impeller sealed off from the motor with out any direct mechanical connection, El Sid pumps are an induction pump that the only moving part is the impeller that is on the other side of a Stainless Steel plate, separated from all of the electronics.
Come to think of it, why not just a small diaphragm pump? much easier to build and on the out side of the tank you just have a coil that pulses on and off with a little iron or magnet button in the center of the diaphragm.