04-20-2011, 11:35 AM
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On the assumption it is diatoms, here is what I posted only yesterday in another thread on this issue:
Diatoms are an unicellular type of phytoplankton. They occur due to any combination of low light, the presence of silicates, excess nutrients and organics, and excess iodine.
Silicates are a type of mineral, about 30% of all minerals are silicates. Quartz, glass, and silica sand (sound familiar?) are some silicates. Silicates may be present in tap water. Iodine is an ingredient found in many aquarium additives. The excess organics and nutrients comes into play mostly in new aquariums where the water has not yet stabilized, and diatoms usually appear before more common green algae which itself will often limit the diatoms.
Maintaining good water quality (regular water changes) is recommended. If diatoms persist in low-light areas it may mean silicate in the tap water, usually 3-4 ppm will cause diatoms; check with your water supply board who should have a list of substances in the water and see if silicate is included. Phosphate removers also remove silicates, as does RO (reverse osmosis).
Live plants help by keeping more stable water conditions. Increasing the light may help, but if the cause of the diatoms is organic/nutrient this will often result in green algae in brighter light. Some snails and "algae" fish like otos eat diatoms.