The Tadpole UV filter is a compact pond system that's fully submersible and doesn't require any plumbing. The Tadpole UV is suitable for small koi ponds up to 1500 gallons and is easily hidden under the water, which is especially useful if you want to add a UV filter after the pond is built.
I wrote a post on a range of submersible UV sterilizers before but the difference here is that this can truly be called a UV filter, as the water is drawn through a fairly large pre-filter by a 500 gph pump, before passing the 25 watt UV sterilizer bulb. This would be ideal for an outdoor temporary quarantine vat for koi because of the ease of installation and when using chemical treatments it could very simply be removed. This is an interesting product that's most likely to be used as a fit and forget UV filter for smaller fish ponds where it will work well removing algae but it also has possibilities as an addition to larger koi systems.