Originally Posted by gatorsfan
No live plants and the brown algae wipes right off.
Then likely diatoms. This can occur in fairly new setups, say during the first 3 or so months [I see you mentioned 3 months]. After that, if what is there is removed, it tends to not return. But if it does, then the causes could be any combo of low light, nutrients and especially silicates [a type of mineral]. Silicates may be present in tap water.
Understand that without live plants, there will always be nutrients present, and some form of algae will use these. Usually it will be a green or red algae, and the more intense and longer the light, the more this will occur. This is not at all a bad thing, as the algae uses the nutrients and produce oxygen, similar to having live plants, and this is good for the water quality and thus the fish. Those of us with live plants are keen to keep algae at bay because it will colonize plant leaves and slowly suffocate the leaf and the plant. But aside from this, it is not "bad" as some tend to think.
So if the light is increased to prevent diatoms, some other form of algae will undoubtedly appear. The green and red algae can be quite attractive, something that I would not say for diatoms.
If you want further reading on diatoms, here you are: Brown Diatom Algae Control | Aquariums Life