Diatoms [most commonly seen as yellow or brownish “algae” in aquaria, usually more prominent in new setups] are single-cell, shelled organisms with cell walls containing silica. They are yellow-green to brown. These organisms have little mobility and live in groups of cells growing a film over the surface of objects---glass, substrate, rocks. In low concentrations they have little noticeable effect in aquarium systems, are always present, and are harmless. They can respond to nutrient increases, growing in abundance when higher concentrations of organic by-products, such as phosphates, are present in the aquarium. However, mostly, they are present more when higher amounts of dissolved silica is present in the water. Silicate, present in all marine aquaria, is dissolved in sea water, and these organisms use it to construct their shells. Under normal conditions, diatoms are controlled in aquaria by the use of grazers such as snails [not fish grazers], and by using an algae scraper to remove the nasty looking film on aquarium glass.
If your aquarium has extra-high levels of silica and excess nutrients in the water, then diatoms will proliferate and quickly grow as unsightly brown films which can cover every exposed surface in the aquarium---including any shelled creatures that you have. Use calcareous substrates, rather then the cheaper river sands that you can buy at the garden or home improvement center. The use of silica-free deionized, distilled, or reverse osmosis water can also help reduce excess diatoms.
Silica is in new tanks at higher concentrations because the sand or substrate, rocks, etc., are new and are made of silica---the food source for diatoms. Once the silica is reduced in the aquarium---as these diatoms will eat much of it, then the brown algae will die back, and the more pleasant and desirable green algae will take its place.
Diatoms are a normal part of the tank cycle. Don’t let them overtake your whole tank, but don’t stay up at night worrying about them either. If you have enormous amts of diatoms even after a few mos, then look for the problem that is causing this---most likely the water you are putting into the tanks. In the meantime, clean up as much of this as you want to keep your tank looking the way you want.
