Ok you may want to click on my name and look up all my threads started. I had the same problem. It looked like free floating diatoms. It got so horrible bad in my tank that it looked like someone pooped in the water. I couldn't even see the fish and it smelled!!! I turned the lights out compleatly for over a week, I did multipal large water changes, I had carbon running, I got marine water clarifier. I got a protein skimmer and it got worse and not better. My whole dinning room stunk form it. I broke down and took apart the whole tank. Got LS instead of dead and added more LR. Emptied the whole thing and scrubbed everything, LR, filters, decor, tank, power heads, EVERYTHING!! I filled it back up with RO water and started again. 3 weeks later the brown started to come back . I almost sold my tank but I got a UV sterilizer and boom! Crystal clear!!! I have never seen it so beutiful. Read my thread and you can see the whole story.