Hi,

diatoms are pretty common in new tanks, but it´s true that they aren´t restricted to them.
I also have some in my ACF-tank which is running since July 2012- without any further problems. Although they´re ugly to look at, I think a low abundance is not a major problem. In the beginning, I also had quite many, but most of them vanished within the first three months. Now, I can successfully reduce them to an acceptable level by scrubbing the walls of the tank every two or three weeks with the regular (weekly) water changes. In my tank, they mainly occur on those parts of the walls that get a few hours of sunlight in the afternoon.