Anaerobic Sand?
 

I found anaerobic sand in my 30G. It was easy to detect because there were parts where my white sand turned black. From what I read, this is bad news to the fish.

My question is... how can I tell if I have anaerobic sand in my tank with black sand?

What causes this stuff? Can it be prevented by raking/stirring the sand or will it keep coming back after the initial outbreak?

Nearly all aquariums with a medium to deep substrate (gravel and especially sand) will have anaerobic regions where oxygen levels are very low or non existent. I believe that 'black sand' only occurs if when detritus gets down to these anaerobic regions resulting in anaerobic decomposition of organic matter.
Have you stirred your sand?
Many people believe they need to stir their sand to prevent this. In fact, you really don't want to stir sand as this creates the potential of burying organic matter deep in these oxygen depleted areas which will then be decomposed by anaerobic bacteria. Anaerobic decomp results in a black foul smelling, almost putrid mess.
Note: with undisturbed sand, mulm collects on the surface of the sand and is slowly processed (decomposed) by aerobic bacteria.

You might remove the black sand this could be difficult and potentially counter productive. I think I might just leave it alone and let nature take it's course...unless you have a lot of it. If so, you may need a tear down to clean the sand.

Moving forward, do not stir sand and use care when planting and/or removing rooted plants - often better to cut and leave roots than to disturb the substrate.
Just my $.02

AD

Wow... that's a lot of really great info. Thanks a lot!

Just add some malaysian trumpet snails. They burrow in the substrate and stir it up somewhat.

Deep sand bed can be used to develop anaerobic bacteria to help remove nitrates from your tank. Deep Sand Beds

So read this article figure out whats going on in your tank and make some necessary changes and perhaps you can get a DSB going and do less water changes. :-D

Otherwise I would just reduce the depth of your sand bed to about a 1/4 to 1/2 inch to prevent the sand from going anaerobic.

It is generally thought that MTS aerate the sand. In reality, they are beneficial waste processors, but require oxygen so they only burrow in the upper inch or so where oxygen is already plentiful.

The absence of oxygen does not make sand turn black. It is the anaerobic decomposition of organic matter that results in black areas. If one is careful not to disturb sand, organic matter can't get to anaerobic or anoxic regions before it has been sufficiently decomposed by aerobic bacteria at or near the surface.

Btw, Just as in SW tanks, I think that [undisturbed] deep sand beds in FW tanks are very beneficial biological filters. Here is another article on deep sand beds one might find very informative.

Is best to vaccum the sand first ,then sift it with a fork,finger's, etc if you feel you must.
Sifting the sand first,, will indeed assure that some of what your trying to vaccum up will get buried.