Rudi, check your pharmacy for IPA, mine doesn't leave any residue. It's about double the price compared to technical grade, but probably worth it. About a year back, I mixed 250ml deionised water with 50ml IPA. To this I added 10 drops of Photoflo 600, roughly 0,5ml. This is very effective and I've used it many times. I always check it before use, but I've never seen anything growing in there. It has quite high IPA concentration anyway (17%), which is a disinfectant. I guess it's about time to discard it, although it looks fine and still smells strongly of IPA.