dark spot in my DSB

[Eduardo cavalcanti]

i have a 29g nano tank which has been running for almost 2 years now. this week i´ve been noticing that the tank is strange and so i found this large dark spot in my DSB. do you know what is it?
PS: i´ve also noticed that my pH is very high, ending the day above 8.6.
here´s an ulgy picture of the spot
http://200.189.178.202/decoders/member_foto.php?SID=348892.4c7e068c5158414a5d78283cacb8d114&TID=00004080241ef9c178b4fce222ab8fda8c41247480&FID=1281351&Original=0

[Desolas]

Looks like a nice black anaerobic zone probably with cyano growing between the sand and the glass, overall it's a good thing. That's where a lot of the denitrification happens.

[55Reef]

It's part of the denitrification process of the sandbed. It's
a good thing.

Here's mine at 2 1/2 years.


http://reefcentral.com/gallery/data/500/2018dsb6.jpg

[ReefSpy]

Hey guys

I been looking for this for months. I actually have 5-6 large spots of this in my sand bed, and it is spreading rapidly. The wierd thing is, it puffs up the sand, sometimes 1-2" above the height of the sandbed. It appears to be an algae, and it is very ugly. I recently replaced my sandbed due to this, adding only 10# of the old sand to seed the sand bed Thinking I had removed all of it in the process).

After only 2 months it is back with a vengance. I would assume this isnt good, but please correct me if I am wrong.

TJ

[ReefSpy]

Oh, I forgot to mention: When I clean the sand it peels up in sheets. The sheets are comprised of primarily sand but very discolored due to the bacteria or algae adhering to it.