I have a decorative pond that is about 200 gallons. It doesn't have any fish or plants. Just purely decorative at this point. Recently moved into the house, so I don't have a lot of experience with this thing. There's a filter and pump that constantly run, so water is always flowing through the filter. I also replaced the liner a few months ago, so that is brand new. It seems to get a nice slimy, green layer on the bottom after a while. So far, I've just added bleach to clear it up. Not much , just a few drops. It will work for a week or two, but then it starts to turn green again. Any ideas on how to clean it? Do I need to scrub the liner to get all the gunk off the bottom? Is there some type of additive that can be mixed in to keep it clear?