02-22-2012, 11:18 AM
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Onemanswarm is correct but with respect to green algae. We're dealing here with diatoms (presumably, as it is brown and easily comes off).
This is common in new tanks for 3 months or so. The common "algae" eating fish will handle this, otos, Farlowella, Whiptails, Bristlenose pleco, etc. But I do not recommend getting any fish just for this, as this can create other issues. I only mention these since if you have them or want any of them as fish, they will handle diatoms.
Once the tank stability is achieved, diatoms should not reappear. If they do, then it may be due to silicates in the tap water and/or low light. Which is why turning off the light will only make this worse. Silicates is more of a concern; silica sand for instance will promote these in many situations, and silicates (a mineral) can be present in tap water. If asked, I can dig out some info on this. But I'm assuming this is a fairly new tank, so this is a temporary issue and I previously suggested methods to deal with it.