It can be a number of things causing it and a number of things you (and "critters") can do to clean it up...
Mostly depends on your view of or goal for "Live" sand..
Here's a C&P from the reef forum...
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The idea is to set up your tank and sandbed so that it is self cleaning, with the help of sand fauna [critters].
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I do have plenty of critters, it just seems like some other tanks look more crisp looking, so to speak!!!
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Well then you need to get those little tiny "ball and chain" sets and put them in a chain gang and then get an itty bitty whip to crack over their heads...Or perhaps purchase or import other "slave" labor to clean your sand...
Then you can stand over them like Pharaoh did and yell "Clean my sand" "Build my Micro cosmic Empire"....
Or...You can get in there and clean it yourself...
On my reef I run BB (no sand) and "command" large ammounts of flow to keep detrius suspended where skimmer and filtration can take it out..What little remains is easily siphoned out from the "visiable" corners it's been "directed" to..Both tanks run a LARGE Fuge with Macros which are "harvested" regularly for mal-nutrient export...
On my FOWLR, I keep Aggressive fish..and they tend to "eat" my (sacrificial) clean up crews...But the shallow sand is nice for asthetic purposes and a few "bottom dwelling" creatures..
So again, good flow keeps most detrius suspended, good oxygenation keeps cyno from appearing, RO/DI keeps Diatoms at bay, and a "bubble vac" sucks out the little bit of uneaten food and junk that tends to settle out...and that's really the only "work" that I do...
My sand bed "may" be dead...But it looks good to me...
PS...I have numerous Mini Feather Dusters growing in my sand as well as Bristle worms and pods crawling under and around the rocks...In a 210 gallon tank I have 1 Hermit crab and 1 Turbo Snail..Both the size of golf balls...Anything smaller is an "empty shell" of it's former self...
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NOTE: I am not recommending "Vacuuming" sand beds on a regular basis to make up for insufficient flow and under sized Skimmers and Filtration...:nope: