I don't do this kind of work nor have I ever had my own pond so I have no idea on this. A friend of mine was asking me if I could help him figure out what to do with a pond his mother has. He told me that she says it turns green even though she has some kind of filter system in it and it does have a plastic liner of some sort. She has even tried pool chemicals in it I guess. It sounds like it is a small pond, maybe like 6'x6' or so but I have never seen it personaly. Do you guys have any tips for keeping a pond clean? Thanks.