I have an Ultima II 20,000 filter on my backyard pond which is about 14,000 gallons. This filter works great, as in filtering, BUT it is way too hard to keep clean. Even while using pre-filters and skimmers, and with frequent backwashing according to instructions, over a season the thing will eventually get consolidated inside into a huge clump of fish-poop-media. !! You can easily tell when the performance is starting to decrease-- when the backwash discharge gets cleaner and cleaner over the season. You might think this sounds good, as your water is getting cleaner and cleaner, BUT WRONG--- the discharge water is getting cleaner because there is simply a channel in the filter tank, and you are mixing up less and less of the media since it has clumped together. They say you never have to clean the media inside these filters---- yes, that's true if you fill in your pond, move away, die, or build another filter! Ha---I've had to suck out all the media in this filter tank and clean the mess 3 times! That is a filthy, filthy assignment. Now my tank is buried, which makes this task VERY messy---and I'm NOT doing it again!! I think this filter would work well for an indoor koi pond, but my pond sees too many leaves, blown in garbage, twigs, etc. The ducks love it and so do the fish, but not the pond owner! I'm constantly trying to keep things clean--- even UV lights and chemicals don't help. What I need is a huge settling tank, but don't have the space for something like that. This burned out pond owner is ready to sell my fish and make a giant bog garden..... Have a nice summer!