If you've been thinking about getting one of those fancy self-filtering water bowls for your pet, skip spending $$$ for a commercial model and build one on the cheap using a fish tank filter.
Animals find interesting ways to muck up their water, whether it's pawing around in the water bowl or switching between eating and drinking so quickly that food ends up in the dish. If you'd like to keep the water dish cleaner longer you can use a small aquarium filter.
You'll need a cheap filter—the cheaper the better, the fancy features like Bio-Wheels and specialised media found in premium filters will be wasted on this project—and a large bowl. You may need to increase the size of the bowl you're using to ensure there is enough water to circulate into the filter; a shallow cat water dish won't cut it for this project. The build is as simple as hanging the filter on the side of the bowl and turning it on but we'd still recommend checking out the full build guide for additional tips like how to shim the filter to keep it from vibrating against the side of the bowl.
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Filtered Pet Watering Bowl [Instructables]