Anoxic live sand?

I just became a member here on 3reef and after doing the whole intro thing, Im going straight to the experts for advice! I just up my first saltwater tank (55 gal) with some live rock, some 'dead rock' and all live sand. The sand is very fine with a homemade plenium and is normally the usual golden yellow color. However, one day I noticed an ashy gray/black splotch against the acrylic. It has very rapidly spread out from the corner of my tank in both directions that I can see. Is this one of those anoxic areas that Ive heard about or is it some sort of fungus or what? I set up the tank at the beginning of this month (December 2008) and cycled the tank in about a week (Ammonia went up to about 1.0). There are no fish in the tank, though Ive noticed all sorts of small critters that came along with the live rock. In the second pic, you can see a 'U' shaped divot taken out of the dark sand. That is where the burrow of a small worm is. It seems to be singlehanded fending off the dark sand by itself. It is for that reason that Im assuming that I have an anoxic area because of the increased water flow that the worm burrow may be providing. Ive also redirected more water flow in the direction of that area of the bottom and Ive noticed a halt in the spread of the dark sand. Im just really surprised by how quickly it has spread through the tank. Literally everday there was a noticable amount of new sand that had gone dark. Any advice, thoughts, comments, etc. would be much appreciated!!

 

well i will be totally honest and say i have no clue what Anoxic means, but i have no clue what is in your sand and it don't look pretty...

 

Oh, anoxic means without oxygen. I know that low oxygen levels are good for denitrifying bacteria but that totally anoxic conditions can lead to bacteria that produce hydrogen sulfide.

 

no idea to what that is really but it sure doesn't look pretty..
if it was me, since there are no fish yet and the cycle is not yet complete, better find out now than later.. i would actually stir the sand and bit and do a full water parameter test after a few hours and see the results.. if the test results show that the water conditions have no been affected, no worries, if not, your cycling time will be extended.. it's better to do that now though than later when your tank is full of life and a nassirius snail or a sand sifting starfish decides to stir up your sand bed.. give it a shot

 

I honestly don't believe that is anything to do w/ Hydrogen sulfide gas... You would see bubbles as compared to "dark" spots. That is most likely trapped detritus in the sand bed.
How deep is the bed, and what grade of sand did you use? How vigorous is the water movement? The Plenum is in the main tank?
This whole hydrogen sulfide thing has been grossly blow out of proportion over the years by the people that are anti-deep sandbed-ers. They have come out with all these stories of how the beds will release hydrogen sulfide and nuke a tank, and equate it to a ticking time bomb. Or another favorite they claim its a nutrient sink, that will like a sponge absorb so much be fore it releases it all into the system again!! I love it! The bed is what PROCESSES the nutrients and completes the nitrogen cycle.
The whole point of you creating the plenum in the first place was to create anoxic parts in the sandbed!!
There was two threads going simulatenous on this site when I first started posting in regards to this exact same subject... I will try to dig them up or you can feel free to browse yourself...
If you are planning on using a DSB or plenum correctly, you DO NOT want sand sifting stars and things of that nature, they are not only disrupting the process but they are eating the critters that are doing the processing!!

Below are some articles that may help you with your sand issue,

How Sandbeds REALLY Work by Ronald L. Shimek, Ph.D. - Reefkeeping.com
The Infamous Detritivore by Ronald L. Shimek, Ph.D. - Reefkeeping.com
Applications of Sand in Reef Aquariums: Theory and Practice by Eric Borneman - Reefkeeping.com

Happy reefing!

 

Thanks for the links and the info! I will definitely check 'em out. The plenium is in the main tank and was made with 3/4" PVC pipe as the skeleton (a rectangle with PVC 'ribs' for support), egg crate (the kind used for ceiling lighting) that is ziptied on and some felt over the egg crate to prevent the sand from falling through. The sand is very fine as I got it from the beach and is 4-5" deep. I dont know if it is detritus because the dark spot has been growing! Also, I have not been adding any food or anything else because there are no fish in the tank. However, any stuff blown off the live rock may have accumulated there.... Im wondering why it happened so fast (the tank was only set up at the beginning of December). Should I just stir the sand and try to redirect more water flow to the area?

 

So rather than stirring up the sand, I wanted to see what would happen if I pointed a powerhead at the area. As you can see in the pics, the dark sand is dissapearing away from the corner of the tank, which is where the powerhead is pointed. Whatever this is does not like added water flow. Any ideas about what this is??

 

Weird! and cool at the same time! i wish i had something constructive to say...

 

Isurus, I have no idea about your sand issues, but out of curiosity, what is your avatar a picture of?

 

lol That seems to be a pretty common question. Its a post larval octopus that is clinging to a peice of plastic. We catch a lot of these little guys; theyre about an inch long.