Hi,
Well, disturbing a deep bed can cause some problems. The majority of these problems are
NOT caused by the release of any particular toxic materials. Rather they are caused by the destruction and subsequent decomposition of sand bed organisms.
There is some accumulation of nutrients and toxic materials in deep beds, where they are being metabolized.
The way to absolutely ensure that these materials get into the areas of the tank which will cause problems is to disturb the bed. Once the bed is set up and functional,
DO NOT DISTURB IT!!!. In a well set up bed with plenty of critters, water movement through all components of the bed is accomplished by animal movement in the sediments and in their burrows. This water movement is sufficient to move nutrients to the deeper layers and metabolized wastes out of them.
This component of the bed depends on stability for its proper functionality, and this functionality is destroyed by "sand-sifting" and disturbance.
When deep sand beds are moved, as has been indicated, there is often a release of nutrients (or put another way, food you added to the aquarium). Under undisturbed situations this food would have been metabolized and removed from the system either as gaseous diffusion or as some other export.
When the pathways for this processing are broken or altered by disturbance, the results of the incomplete metabolism -the sulphides and nitrates, make it back into the upper layers of the tank.
Moral:
DISTURB THE SYSTEM AS LITTLE AS POSSIBLE111
Cheers, Ron