Originally Posted by fishkid
Just add some malaysian trumpet snails. They burrow in the substrate and stir it up somewhat.
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.
Originally Posted by goldfishyman
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.
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.
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.