TetraPond Bio-Active Pressure Filter is a great choice for garden ponds and water features. They come in three sizes to accomodate water volumes of 1500 to 4000 gallons. Optionally each model is available with a pre-installed UV clarifier of 9W or 18W depending on the model. The new design can comfortably work with water flows much greater than the previous models.
Aqua Ultraviolet's Ultima II filter series is a third generation design. We recommend this filter for garden ponds and water features, as well as koi pond when properly sized. The unit combines both mechanical and biological filtration. When properly backwashed, a simple procedure, it will never need to be opened for cleaning. For ponds from 1,000 to 100,000 gallons.
Evolution Aqua's reputation speaks for itself.
Their koi pond filters are very well known amongst hobbyists and are probably the best money can buy. They are simple, yet rugged, highly efficient, and the only ones we know to return water to the pond with more oxygen that it had coming into the filter. All that combined with easy low maintenance certainly merits strong consideration.