UV STERILIZERS AND OTHER WATER CLEANING DEVICES
For most ponds, mechanical and biological filters will be sufficient to keep your pond clean.  A UV sterilizer should be considered when algae and/or fish diseases are an ongoing problem.

  • Ultraviolet Sterilizers.  Ultraviolet  (UV) water sterilizers are sometimes used with a biological filter.  These UV sterilizers are connected into the inlet side of the filter.  The unit itself consists of an ultraviolet bulb (UV) enclosed in a sleeve that is transparent and waterproof.  Some biological filters also use a UV sterilizer like the one shown at the left, and it is placed inside a tube plumbed into the system.  Microscopic organisms exposed to the concentrated UV light are killed as the UV energy causes their cell content to explode.  Fish parasites, algae, viruses, and bacteria can be killed in this manner.  Another benefit of using a UV light in conjunction with a biological filter is that it encourages minute particles to clump together and become trapped.  UV sterilizers usually are not needed with a biological filter, but if fish diseases or parasites are an ongoing concern, they may help.
  • Ozone Generators.  Just like the UV sterilizer, the purpose of these devices is to purify the water and reduce bacteria and fish parasites.  They convert oxygen from the air into ozone and infuse it into the water, where it breaks down nitrates, phosphates, ammonia, and chloramines into harmless gases that escape into the pond.  The ozone, like the UV light in the sterilizer, encourages the clumping of toxic waste particles so they are easily trapped by other filters.  Ozone generators are becoming increasingly popular with koi hobbyists after being used for years to sterilize swimming pools and spas.  These are costly units, but if you want a koi pond that is crystal-clear with few or no plants, they may make a good investment.
  • Natural Plant Filters.  Ironically, natural plant filters are another option for ponds with few or no plants.  A separate small pond or a large tub make up this filter.  They are densely planted with such plants as water hawthorn, watercress, water lettuce, or water hyacinth, place between the filter outlet and the main pond.  Algae growth in the pond is reduced because the plants consume nitrates and other nutrients produced by the biological filter.  You can also incorporate them into your overall landscape design.

Using natural plant filters to remove excess nutrients is a low-tech, relatively inexpensive option.  I used it it my pond when I had severe problems with algae and I now have water that is crystal-clear.  The expense of UV sterilizers and ozone generators, while an option for the serious koi hobbyist, probably can't be justified unless you are raising valuable fish.