Henry, I suggest a Maxi Jet 1200. Just make sure the rubber O ring on the pump forms a nice tight seal. Jim N. and Tim are using Mag three's I think with equal success. And if you use ESV kalk exclusively, your pump will last indefinately. Ive been using ESV and the same Maxi 1200 for over a year, I took it apart to clean it recently and the pump looked brand new. I dont recommend using pickling lime or the like in these types of reactors.
Jim