Pond algae comes in many different forms, even different colors, green, red and brown. The two kinds of pond algae frustrating pond keepers are green water algae and string algae or blanket weed. Green algae are single cell plants that make the pond water look like pea soup. These plants are so small they can pass through a pond filter. String Algae often grows in water with high pH balances. The pH must be lowered between 6.8 to 7.4 to slow the growth of this type of algae. When algae becomes overabundant the decaying algae depletes oxygen levels. During the summer oxygen levels may decrease, causing problems for aquatic plants and fish. The Water Wych is a natural algae inhibitor that has been developed and used by the world famous Wychwood Waterlilies for more than 10 years. The Water Wych absorbs excess nutrients such as phosphate and nitrate from the water which together with sunlight, makes algae grow. It also inhibits string algae or blanket weed. The Water Wych is a totally natural product and is Wychwood's naturalistic approach to water gardening. Made from a special blend of volcanic rocks, carbons and silicates, it is completely free from any manufactured chemicals. The Water Wych does not need any device to make it work. However, if placed in a pond with a pump and existing filter, it will work even faster! Simply immerse the whole unit in the water and let the magic do it's work! To obtain the best results from your Water Wych the water in your pond or water feature should be always maintained at levels below pH 8.0. Of the few customers who have reported poor results using the Water Wych, it was due to consistently high alkalinity levels - pH 8.5 and above. Reduction of the alkalinity to a level of pH 8.0 or lower results in the Water Wych becoming totally effective within a few days. Then, within 2 to 4 weeks, the water will become as clear as a mountain stream! The Water Wych needs no maintenance. Once the contents have been saturated, remove it and replace with a new one. The contents of the saturated unit are 98% recyclable. For best results, the Water Wych should be replaced every 6 to 9 months to ensure algae-free water at all times. 4 Sizes available: Mini 9" x 9" x 4.5" - Ponds up to 840 Gallons Small 8" x 8" x 8" - Ponds up to 1800 Gallons Medium 9" x 9" x 12" - Ponds up to 3600 Gallons Large 11" x 11" x 16" - Ponds up to 8400 Gallons