I could possibly run the smaller UV I have on the barrel of rainwater for a few days before doing the water change. I'll see how they look when I get back from holiday with only tap water for changes. Hopefully its not anything leeching from the background, I'm counting on the waste being greatly reduced so they only need a water change after two weeks (instead of one week as usual when I'm feeding heavily every day), but if there is something leeching then its going to build up a lot more with only half as much water being changed.