If it is continually clogging up with green algae, you can put in some very small "straw bales" which are very tiny packs of straw available from any Acquatics shop. I'm not exactly sure how they work, but once they're in the water and really saturated, they emit some substance which helps to keep the water clear. It costs around �4.99 for a pack of two bales, and the effects last several months. You need a very small amount of straw for the average size pond, probably no bigger than the size of two loo roll centres joined end to end, which is what these mini straw bales look like.