I think the water gets increasingly smelly if it isn't used. I have one butt in an awkward position which I use less often and that water tends to smell when I draw it off. I don't think the plants mind though so I just grin (or grimace) and bear it!

Sorry, I can't answer your question, but I just loved the way you asked it !

If you are around when it rains heavily run some off. Anorak on hood up, and get some exercise. Sling it on the most needy shrubs and bushes etc. I do that with my Camellias this time of year. It may look crazy but the camellias bud up well as they like water at this time of the year when it can get very dry.

I saw on the internet that ceramic chips help keep the water fresh. I haven't tried them myself so not sure how effective they are. Here's the link where I found them:

http://www.harrodhorticultural.com/em-ceramic-chips-pid7955.html