Pool filter sand turning black underneath layer?

[adr_93]

Im in the process of moving into my first apartment and i saved the tank for last. Im moving the 75gal tank in which I have had pool filter sand for about a year or so. Now I have moved around the sand before because I know it traps bubbles. But im just about to get the last bit of sand out and I noticed its turning grey or black. Didn't know what it meant. If its dirty or what. Anyone experience the same? Just want to make sure if I should use it again or not. I havent had any problems with the sand other wise.
Thanks.

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[adr_93]

My pops said it because its getting old. Its darker in person than in the pic.

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[Gill Blue]

it's more than likely where something organic was in the sand and decomposed anaerobically.

 

[wrslrcichlid]

Gill Blue is correct.

 

[Jhncf]

Did you have live plants in the tank? This used to happen in my planted tank if roots rotted under the sand.

 

[divemaster99]

That happens to me under plants and under driftwood.