We have a little landscape pond, just like 6x3' or so. It MUST have a pump and fountain or other aerating. Other wise it will just be a green glob of stagnant water and smelly and full of mosquito larvae. Even with the pump and fountain, it does take some care. It helps to have it shaded; the algae loves sunshine. Our pond is doing better since the lilac bush next to it got bigger, so it is quite shady. There is blue stuff you can add that is artificial shade -- makes the water less transparent to sunshine. We found that it helps just to pull out say 1/4 of the water, once every week or two, and replace it with fresh. If you don't have fish or anything in it, you can use hose water with chlorine for the replacement, which also helps. Otherwise use rainwater or water that has stood for awhile to outgas the chlorine. Changing the water like that is a tip I got from the aquarium folks. A small pond is basically the same idea as a large aquarium, I guess.... Raining into the pond isn't a problem.