10-02-2012, 07:13 PM
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Algae will naturally occur in any environment under water in the presence of light if nutrients are available. And plain old tap water is full of organics. The intensity of light will determine the type of algae, and in low light the brown (or more correctly, diatoms, not a true algae) will be prevalent.
Diatoms often appear in any new tank, usually during the first 2-3 months. After this they should be gone. I won't go into what can keep them around, as that is not directly relevant to your question.
Clean the diatoms off as best you can, but if you don't have live plants, this is not going to harm anything, including the fish. But if plants are intended, diatoms will multiply on the leaves and smother the plant, just as any green or red algae can, so that is why we aim to keep algae of any sort under control. But without plants, algae of any type is natural and actually good, as it is performing some of the same functions that plants do, namely using nutrients including toxins like ammonia, and producing oxygen (the green types).