Latin Name: Diatoms
Common Name(s): Brown Algae
Few objects are more beautiful than the minute siliceous cases of the diatomacaea...Charles Darwin
Although diatoms are commonly (and mistakenly) referred to as brown algae, their closest relatives are golden algae. The skeleton (cell walls) of a diatom is made of hydrated silica embedded in an organic matrix. Each wall is composed of two halves that fit together like a shoe box and lid. Some diatoms can move around by squirting stuff out of slits in their cell walls. There are about 10,000 known diatom species.
The photosynthetic pigment of diatoms is brown. In the summer waters of a healthy pond, diatoms can grow to such numbers that submerged plants can have the appearance of being covered with a brown mud which the microscope reveals as a dense growth consisting entirely of diatoms.
Source: Chuck Huffine on APD 10/01
How to Treat:
The most common myth about diatoms is that they are caused by a certain light level (some say low, others high) or just old bulbs. In my experience, diatoms grow in all light levels from low to high whether the bulb is old or not. Diatoms, like all algae plagues, are invariably caused and sustained by excess nutrients in the water column. As Kevin has already pointed out, silica is the most important of these in the case of diatoms. However, removing silica, which can cost serious $$$, is not necessarily the best method for tackling diatom infestations.
IME, the two best solutions for diatom control are Otocinclus catfish or simply waiting them out. A half-dozen healthy otos will clear a medium-sized tank (~55 gallon) in a few days and keep it clean
after. Waiting the diatoms out means scraping them from tank surfaces and letting the filter and water changes remove them from the water column which removes both the diatoms and consequently silica (and other consumed nutrients) from the tank water. Eventually, the plague will peak and then fade away rather quickly. For the impatient, there are silica removal resins that will take the silica out of water, but they can be a pain and are not cost effective. Or you could use a reverse osmosis filter to remove practically all minerals from tap water, but RO can cause as many problems as it cures in some cases and it is certainly expensive both in time and money.
At least one study have shown that diatoms store phosphate in sufficient quantities to provide for 100 subsequent generations.