That sounds like brown diatoms to me, as Pogthefish suggested.
Brown diatoms are a very common occurance, especially in newly set up tanks.
They should eventually go on their own. It appears in a sudden bloom and eats excess nutrients in the water. It will die off once those nutrients have been removed or used up.
Best way to get rid of brown diatoms is, high light, frequent water changes in order to remove excess nutrients, as well as refraining from dosing if you are, say for plants.
In a planted tank, the plants will eventually out-compete the diatoms and they will die off. Although sometimes some plants will die as the diatoms appear in a bloom and can damage some weaker plants by either stealing too many nutrients and/or growing on said plant to the point where it cannot get enough light.
You can remove the visible diatoms on the glass with a scraper and such, but will go away in time either way.
Ottoclincus catfish love eating this diatom in particular, so if you have enough room and wouldn't mind an otto, they make a great addition to your cleaning crew. IMO they are the best for this diatom, although rams horn, nirite and some shrimp are known to eat this one as well. Plants also eat the nutrients that this diatom eats, so plants may also help.
So in short.
This is normal.
Ways to get rid of...
-High light ( This diatom has a tendency of growing in lower light sections of tanks)
-refrain from plant dosing
-Manually remove some of it ( for aesthetics)
-Frequent water changes
- introduction of cleaning critters
-Go with the flow. As time passes it will also pass
Hope this helped.
Last edited by PixelatedPaint; 08-11-2012 at 10:15 AM.