If you use distilled water you should have no impurities, distilled water is as clean as you can get. I use to have problems too with negatives getting full of stuff you don't want to get in the first place. My solution was simple, went to a hardware store, bought a long thick PVC tube (the ones they use for drainage and toilet connections) bought 2 end-caps you can take off and a few metal pieces to make the feet. I drilled a few small holes in the bottom and the top to vent the evaporating water and with a copper wire I made a system so I can hang my negatives. Since then I have virtually no contamination any more and it's cheap to make.