Have you tried killing them before putting them in? I just cut mine in half. This doesn't stop movement just stops them from thinking. It is definatly a good idea to put the worms in moist peat for about 2 days before putting them in, as it cleans them out. I don't know about worms that survive to the substrate mess up the water, but I would assume they can't really survive in water. If I'm wrong please correct me, as I have been wondering this as well.