So exactly when do diatoms appear, other than during a tank cycle ? I only ask because my tanks been up and running for over two years now and I recently added some live rock into the tank that has been "cooking" since winter in the basement tank. And now its covered in diatoms. The only difference is the tank downstairs has no light on it, just a few powerheads, heater, and crushed coral for the substrate. No fish/corals/inverts in this tank either. So I would assume the rock has cycled long ago. Could the light be causing the diatoms to appear ????? Or should I be doing some water tests. Everything else is normal.