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Brown dust constantly forming on the rocks and sand. Changed all the filters in my rodi, doing constant water changes and sand cleanings, I'm running out of options 
Is it stringy, easily removed from rocks, CUC won't touch it, little bubbles in it, seems to be less in the morning then it's almost covering everything by the time the lights are about to go out?
The two rocks in the last picture do it appear to me to have been wet for eight months. Are they later additions?Tank is 8 months old. This has been happening for 3 months, killed a bunch of coral because it's relentless. I cleaned the sand yesterday and it looks like this today already. I scrubbed the rocks yesterday and blew them off today so they are looking ok in these pictures. 90gallon tank w/sump, doing 5 gallon changes every two days to clean the sand, and if it starts to take off I do a 15 gallon change once every other week.
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Look close. He's got everything. Normal.I think you have dinoflagellates and not diatoms.
The two rocks in the last picture do it appear to me to have been wet for eight months. Are they later additions?
What is your cuc comprised of?
Look close. He's got everything. Normal.
Check out that link. I would reccomend turbos , turbans , and astreias. Rock grazing herbivores.I was in there with a tooth brush yesterday scrubbing the rocks to get the brown off, while I was doing a water change. They may look newer because those rocks are easier for me to scrub with more force, so I probably got more off of them.
My CUC is trochus and nassarius snails (although my hoeven wrasse killed like half of them in the last 3 months), 2 emerald crabs, 2 hermit crabs, algae tail spot blenny.
Oh and the new rock thing. The embedded algaes and precursors to corraline aren't showing as much. So new additions can add to the dissolved minerals from the surface of the rock. Silicates and more actualy. One of othe the fuels for mirofaunal growth in addition tonnitrigen and phosphate and light energy.I was in there with a tooth brush yesterday scrubbing the rocks to get the brown off, while I was doing a water change. They may look newer because those rocks are easier for me to scrub with more force, so I probably got more off of them.
My CUC is trochus and nassarius snails (although my hoeven wrasse killed like half of them in the last 3 months), 2 emerald crabs, 2 hermit crabs, algae tail spot blenny.
There is a balance between phosphate and nitrate, I imagine you are reading 0 in both. You can try to bring your nitrate up to around 2-3ppm to help combat it.
What are your readings? It doesn't look like diatoms not dinoflagellates to me.
Look close. He's got everything. Normal.

