In the picture, it is the brown stuff that I am trying to ID. It just started in my tank around last Wednesday. It was about 4 days after adding 20 pounds of live rock and two 24" T5 lights (1 actinic, 1 10,000K). I was using just regular tap water for everything until I read about all the negative side affects of doing this. I now use RO/DI water for top offs and water changes. PH: 8.2 Ammonia: >0.25 ppm Nitrite: 0 ppm Alk: Normal (between 1.7 - 2.8 Milli equivalents per liter) SG: 1.025 I didn't test for nitrates and I can't test for anything else right now. I just added the power head today for increased water flow. The picture with the red mushrooms in it is what my sand bed used to look like. This brown stuff is growing only on the sand and the rock that doesn't have coralline algae on it.