For my cat I have a Drinkwell water fountain bowl that I use. I've used it for a few years and have no complaints with it, and it provides a constant stream of fresh water with its additional reservoir attached. Lately, however, I've noticed that after I clean it and refill it, within a day or two green mold starts to grow in the main bowl area. I then have to clean it and refill it, only to have the mold come back again. This began happening within the last two weeks. Just an FYI--I moved the water bowl along with his main litter box and his main food bowl onto my outdoor balcony, which is a huge, deeply recessed and covered balcony. I assume the mold build-up is partially because the bowl is in an outside environment, and the Houston area can get a bit humid, to say the least. However, the water bowl has been on the balcony for at least 2 months, and it has been humid off and on during those times, but not terribly humid. The mold just started popping up; it wasn't there when the bowl was first moved outside. I know there are water treatment items for such situations, but they're designed for other entities and not for drinking water for animals--or are there? I've looked around and can find only items that are designed for fish tanks. Even a Google search couldn't find something that could be safely added to the water to prevent or delay mold build-up while be harmless to a cat drinking that water. If anyone has any ideas on how, or even IF, I can tackle this minor issue, I'm all ears.