I manage a concrete pond system for a mini golf course, there are two large ponds connected by a concrete riverbed with a fast flow. The bottom pond is about 15,000 gallons and the top is probably about half that. The plumbing is all underground, and there was never a filter or a place to put one. The last manager just kept the ponds lifeless and cleaned them out once a month. I would like to fill them with waterplants and maybe some fish eventually. I was thinking that I could fill the riverbed, which is about 40 feet long, with lava rocks and weigh them down with a layer of larger rocks to prevent them from washing away. Assuming that I can keep the rocks in place, I think that they will become encrusted with bacteria and serve as a very long biofilter. Does anyone have any experience with this sort of system, or is there some reason this won't work? Any advice is greatly appreciated.