Sand substrate turned black in an area

[fishb0ne]

Like the title says, in my brackish I've got some pool filter sand which in one spot, about the size of a closed fist, turned into a blackish color, kinda like as if activated carbon dust got in it. On the surface it was nice and clean but as I dug and dug I got to the bottom of my tank and it was mixed with this stuff. I sifted it around a bit and now it seems to be fine, I mixed it with the rest of the stuff. My filter is on the opposit end. What could this stuff be?
Water parameters are all fine, I've heard of pool filter sand discoloring, but blackish?

 

[bs6749]

Have any live plants in there before? I had some sort of aponogeton in my 55 gallon tank that had pool filter sand and the buld of the plant started to rot away. The end result was a spot of discolored sand (nearly black) and the area smelled like sewer. The plant smelled bad so I threw it in the trash. Could also be some sort of algae I suppose.

 

[Devilishturtles]

Black spots in the sand are indicative of a hydrogen sulfide gas buildup that grows in anaerobic conditions. Stirring your sand periodically will keep that from happening, as well as making sure it's not packed down too tight. It's not usually dangerous to fish unless you have a LOT of it in your tank. Stir sand during water changes and you'll usually prevent it from building up.

 

[bs6749]

That would second what I said because the H2S gas would smell like sewer or rotten eggs due to the sulfur in the compound. I also noticed a rather large bubble come up from the area in my tank where the plant was after I stirred it.

 

[Zagz]

Definately the gas building up. I have encountered this myself before. As mentioned stirring regularily will prevent this problem.

 

[fishb0ne]

That's what pretty much sounds like it is, sorry for the newbie question, it's just that I've never seen one. Since then I've removed half of the sand I had in the tank, I don't know what I was thinking when I put it in but I really had a lot. Now I have a substrate that's about a finger thick and it still looks nice and enough to cover the glass bottom. I'll use the turkey baster of life [I really use it for a lot of things in my aquariums] to turn the sand every now and then and I'll do it more often than once a week since that doesn't seem to be enough.
Nope, no plants in that particular area. I regularly drop food in that general area but that's about it. I'll try again to acclimate some malaysian trumpet snails to the brackish water, last time I did so I ended up killing them. I hear they're awesome at burrowing through sand although they breed like crazy sometimes. The red claw crab should take care of most
Thanks everyone!