I could lend you my cats.....they seem to think the Christmas tree resevoir is a classy water bowl for them! We end up topping it up every other day because of them, it would never get a chance to stagnate!
On a more serious note, you could try some charcoal in the water to keep it sweet, and make sure you have removed any stubs of branches under the water level that might rot. Or try a few drops of bleach in the water?