I have been told diatoms appear at the end of your tanks cycle but I had one tank that had them and another that didn't. Have you gravel siphoned it out?
Tanks been cycled a good 6 months. Live rock and everything else has been in use for about a year it's just an upgraded tank. I have vacuumed and it just came back. I'll up the flow after lights out tonight an try not to stress my livestock.