UV is a waist of time and unsightly for such a large, beautiful, natural pond. Great video btw. Do you have gravel and rocks in the pond? This will help w/ providing a bed for bene. bacteria. Although the falls should provide enough surface area, if there was gravel in the pond it will only boost the bacteria capasity. You might not want to hear this, but I would lose all the plants in the filter. Transplant them into the main pond. Use the filter as an area filled w/only Water Hyacinth. They are unbelievable in growing fast and using up extra nutrients in the water column. You will have to buy them each year, but a dozen or so will produce thousands by the end of summer. I use 5 or 6 for my 1200 gal. pond and throw away or compost 20-30 plants each week in the peak of summer. Like mystic said, plant more lillies in the pond. This will help shade the surface since it looks like it gets hammered by direct sunlight all day(no near buy trees). Do you fertilize your lawn or plants in the pond? Being on a slope, run-off will allow fert. and other chemicals to possibily enter the pond. No need to fertilize the plants. A heads up for spring, algae bloom for sure. It happens every spring/fall and is inevitable but can be slowed by a good fall clean-up. No trees right around the pond but I can see nearby woods that the wind may carry leaves. In the spring just keep up w/ maintainence and by May1st get a ton of Hyacinth and watch the green water disappear. It can also be the case of a fairly new pond. With small ponds 1-5K gals. The first year is OK. The second year, which is just like a kid in the terrible twos, the pond is pretty unstable and still getting established. The thrid year is great and so on and so on.... Your pond being so large 24K+ gals. It still may be young and getting established. Again the beneficial bacteria and other organisms need some time. Next fall try a cold water bacteria booster that will introduce different types of bacteria that will grow in cooler water temps. Use it again in early spring, even this year, to jump start the pond. Oh yea welcome...
