Brown spots on new tank, just checking

[cps68500]

New tank, several weeks old. Only base rock, crabs and a few live rocks now. Just starting to get foam in skimmer. This week I've notice a LARGE amount of brown all over my rocks, and now spread to other items in the tank, as shown on my powerhead.

I'm hoping it's normal, but every tank is different. I've read elsewhere this could be caused by tap water start... (i'm already looking at RO/DI from airwaterice.com)

 

[jimrawr]

Looks like the begining of a Diatom bloom, which I would say is normal and will go away on its own. It will get much worse than it is right now before it gets better though. Also stop using tap water asap as you will have far more and worse problems than a Diatom bloom..