Dark circles in my DSB

[cgr]

I switched from CC to DSB about four months ago and I am really happy with the results. In the past two weeks these dark spots have appeared at different depths of the sand bed. Any idea what it could be?
120 Gal FOWLR
1 Yellow tang
1 Blue tang
1 Sailfin tang
3 chromis
1 cleaner wrass
1 tomato clown
1 sleeper gobie
1 scooter blenny
1 bicolor blenny
1 coral banded shrimp
1 sandsifter star
emerald crabs, snails, blue and scarlet leg hermits
Wet/Dry filter w bioballs
5+” DSB
Ammonia 0, Nitrites 0, Nitrates undetectable,
PH 8.2, Salinity 1.024

 

[jeo]

I don't know exactly what they are but thats normal. I have all different kinds of spots ect in my dsb. I think they are just caused by the fauna in the sand -a good sign of a developing dsb.

 

[cb]

Your sandsifting star will deplete all the good stuff out of your sand bed.: