"Brown algae" (diatoms) This is often the first algae to appear in a newly set-up tank, where conditions have yet to stabilise. It will often appear around the 2-12 week period, and may disappear as quickly as it arrived when the conditions stabilise after a couple of months. It is essential to minimise nutrient levels to ensure the algae disappears - avoid overfeeding and carry out the appropriate water changes, gravel and filter cleaning, etc. Limiting the light will not deter this algae, as it can grow at low lighting levels and will normally out-compete green algae under these conditions. If brown algae appears in an established tank, check nitrate and phosphate levels. Increased water changes or more thorough substrate cleaning may be necessary. Using a phosphate-adsorbing resin will also remove silicates, which are important to the growth of this algae. However, as noted above, it is essentially impossible to totally eliminate algae with this strategy alone. Due to its ability to grow at low light levels, this algae may also appear in dimly lit tanks, where old fluorescent bulbs have lost much of their output. If a problem does occur, otocinclus catfish are known to clear this algae quickly, although you may need several for larger tanks, and they can be difficult to acclimatise initially. There are some very plausible theories as to why this algae often appears in newly set up tanks and then later disappears. If the silicate (Si) to phosphate (P) ratio is high, then diatoms are likely to have a growth advantage over true algae types and Cyanobacteria. Some of the silicate may come from the tapwater, but it will also be leached from the glass of new aquaria, and potentially from silica sand/gravel substrates to some extent. Later, when this leaching has slowed, and phosphate is accumulating in the maturing tank, the Si:P ratio will change in favour of phosphate, which is likely to favour the growth of green algae instead. http://www.thetropicaltank.co.uk/algae.htm