I have a farm type float valve (~$20) fed by the garden hose that keeps my ducks' water bowl filled, but it doesn't keep it clean. I bought a submersible dirty water pump (~$50) and plugged it into a programmable light timer (~$10). I programmed it to turn on twice a day for 1 minute. That drains the whole bowl and then some which keeps the water fairly clean and lets us leave them for as long as their feeder will allow and it means we're not out there twice a day changing their water, which is a great thing. However, it isn't perfect. It's a little pricey for starters. For reasons unknown, the ducks will occasionally put 1-2" diameter rocks in the bowl that can clog the pump and require a clean out. It doesn't work in winter unless you have some way to keep everything from freezing. Pyrite heated garden hose would work, but that adds even more cost. And most recently, after about two years of operation, the pump motor is starting to wear out, so I'm going to have to replace the most expensive part or come up with a new system.