The Decorative-Pond Algae Control in a Line Filter

Algae Blooms

• Algae cause seasonal water-quality problems in decorative ponds, becoming abundant as water warms. Ponds with a perfect balance of animal and vegetable life stay clear without treatment or filtration, according to the Alabama Cooperative Extension System. Finding that perfect mixture of fish and aquatic plants might be impossible for the average homeowner, since many nutrients enter the pond from outside sources. Storm runoff entering the pond can carry fertilizer from the lawn as well as yard debris, adding to the nutrients already in the water. Algae blooms darken the water in response to an overabundance of food.

Filtration

• Circulating the pond water through mechanical and biological filters removes some of the excess nutrients from the pond. Mechanical filters remove suspended debris, and biological filters draw the water through layers of bacterial mats. Bacteria in the mats digest the extra nutrients, and cleaner water returns to the pond. Aeration by pumps also places more oxygen in the water, causing phosphorous to settle out of reach of suspended algae. Microscopic algae need light as well as fertilizer, and only flourish in the upper few feet of pond water. Filters can reduce algae populations by limiting nutrients, but don't remove algae from the water.

UV Sterilizers

• Ultraviolet sterilizers added to the return line of either above-ground or submerged filters kill algae and bacteria in the water flowing back to the pond. Not all UV sterilizers work underwater, however, and installing submersible types incorrectly can generate dangerous electric currents. UV sterilizers work by channeling water at a controlled rate through a small chamber holding a long quartz ultraviolet bulb. Water stays in the chamber long enough for the light to kill plankton. The algae numbers decline gradually, preventing the sudden drop in dissolved oxygen that can kill fish when algae blooms expire suddenly. UV sterilizers leave some algae in the pond.

Matching the Pond

• The wattage of a particular in-line UV sterilizer determines the volume of water the device treats effectively. A 40-watt sterilizer treats a pond of 1,200 to 2,000 gallons if combined with a pump-and-filter system that moves 3,000 gallons per hour. A 240-watt sterilizer matched to a 12,000-gallons-per-hour pump and filter controls algae in a pond holding from 13,000 to 17,000 gallons, according to Aqua Ultraviolet website. UV filters only reduce the numbers of microscopic algae in the pond. Floating algae mats or beds of filamentous algae might grow better in the clarified water.