I have a Mr Turtle sandbox as a duck pond for 8 ducks. It gets very dirty very fast. When i was dumping and refilling daily, it made such a mess of the yard that i put on my thinking cap and this is what i came up with...first, my sweet hubby dug me a hole about 8 feet long, 4 feet wide, and about a foot deep. Then we got a few truckloads of rocks. They're not large rocks, but not tiny either. This is the drainage area. I then contacted my plumber to see if he would put a drain in the pool. I told him i wanted a drain with a shut off valve so i didn't have to dump the pool to drain it. He said he would help me out. The next day he brought me a valve, an elbow and a piece that goes on top of that, which was threaded, and had a metal nut on the top of it...along with a rubber washer...or maybe it was a gasket. He told me to buy plumbers cement, and explained how to do it. So a little nervous, i drilled a 1" hole through the plastic bottom. (he warned me to drill it with the drill in reverse so it didn't crack the plastic) After the hole was in, i put the plumbers cement on the bottom and top, and edges of the drilled hole. I put the threaded piece through, put on the washer, then screwed on the nut. The elbow was already connected to the threaded piece. I connected a longer piece of pvc pipe to the other end of the elbow. After it dried overnight, i did some measuring, and dug a trench in the dirt, just next to the drainage area, but inside of the ducks pen. I layed the pool over it, laying the whole drain in the trench, adjusted it so it sat properly, and filled it with water. I drain and refill it at least daily, sometimes twice daily. In the winter, i leave the drain open, and when the hose is not frozen, i'll give them swimming water in the pool...provided i can get water out of the hose. I don't leave it in the pool overnight, as it freezes and then you have to wait for a really nice day for it to thaw so it can get out. Ducks don't mind, they like sitting on the ice! The dirty water drains into my drainage area, leaving the rest of my yard in good shape so we don't have a muddy mess all the time. If it gets cold, sometimes the valve is really hard to turn, as it starts to freeze...so in the winter, i only give them swimming water on decent days, when i am around to drain it a little while later, after they had time to swim.
I am going to attempt to install drains into two more pools very soon, as my geese need a place to swim as well! If only i knew what the part with the threads was called....