04-26-2010, 07:57 AM
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Diatoms would be my guess too, but like Byron said, without something visual it's hard to say.
If they are diatoms, it's usually a sign that you're starting to get some nitrogenous buildup in the tank, and hence, the environment is becoming more algae friendly. Green algae outcompetes diatoms for nutrients, so if green starts up, the brown usually goes away. But, then you have the green to deal with.
Do an image search for diatoms, and compare that to what you see in your tank. If that's the stuff, check your water for nitrate / phosphate buildup, as that's usually the cause.
On the plus side, the presence of diatoms = thoroughly cycled tank. Heh.