All living things produce phosphates as biological waste. Almost all aquarium water contains some level of phosphates. In very low levels, phosphates generally don't cause problems. However, phosphates can encourage the growth of nuisance algae, causing unsightly algae blooms. You can control phosphate in a number of ways.
Phosphates are inorganic salts of phosphoric acid. All multicellular things, including plants and animals, produce phosphates. However, plants and algae use phosphates for nutrition, so they use more phosphates than they release. You can find phosphates in just about any biological system. They enter aquariums a number of ways.
Phosphates basically act as a fertilizer for algae. Both algae and higher plants need phosphate in order to thrive and grow. If you have levels higher than 0.1 ppm of phosphate, algae can easily bloom or overrun an aquarium. Depending on other aspects of water chemistry, different types of algae may dominate the tank, including red, brown and green algae. Some algae grow along surfaces, while others float in the water column, causing the water to turn green and cloudy.
Sources of Phosphates
Overfeeding and overcrowding your aquarium both contribute to high phosphate levels. Also, some municipal tap water contains enough phosphates to cause problems in aquariums though the levels are harmless to humans. You can test a sample of your tap water for phosphates to rule this out. Additionally, if you feed grocery store seafood to fish, it may contain extra phosphates, since some grocers add phosphates to these foods as a preservative.
You can control phosphates a number of ways. First, try to figure out the source. If your phosphates come from extra food, feed less. You should feed only the amount of food your fish can finish in 30 seconds. They will usually beg for more, but any food they don't finish will usually rot, raise phosphate levels and cause other problems. If your tap water has phosphates, try running it through a reverse-osmosis unit to remove them. Well-maintained aquarium plants can absorb phosphate more efficiently than algae, limiting algae growth. However, keeping aquarium plants is harder than it looks; use this approach only if you know you can keep them alive and healthy.