I use a stock tank; the low sided one made for sheep. It would take a heck of an expensive filtration system to keep duck water clean. I just fill the tank, let them play for 2-3 days and then drain the tank and let it sit dry for 4-5 days. I've got a hose on the drain and when I drain the tank, I run it around to the various trees so the water doesn't get wasted.
Also, I don't fill completely. Just deep enough to swim. Water is expensive here.
With the drain, the stock tank is a lot easier to drain than a kiddie pool. I also keep several Rubbermaid storage bins filled with water. Those get cleaned every day. They are easy to tip out and the ducks get to have their bath, although not swim.