When your LFS
guy referrs to "contamination" he is referring to Old Tank Syndrom (OTS). This deals with two different possible faults of the DSB
. The first is saturation. Some believe that it can become saturated with heavey metals and other contaminants and eventually start leaking them back into the tank. Its kind of like if you put a large sponge on the table and started dripping water into it. Eventually the water would start running out onto the table. There is no hard evidence that this exsist.
The 2nd concern is what is called Lump Syndrome. Due to a variety of factors (which can be avoided) the DSB
is not regularly shifted and begins to develop solidified lumps of sand. This happens when the infauna (pods
, worms...critters in the sand) are depleated or are not able to susstain a population with in the system. Lack of infauna would mean that the sand is not "moving" and has become stagnant. Possible cause could be a Orange Spotted Goby who has sifted them out of the sand and decimated the population to the extent that they will not recover untill the Goby is removed. At that point, because the sand is not being shifted, the sugary excretions of the bacteria present in the sand will bind the grains of sand together and start to form lumps. These lumps trap sulfides. When one of these pockets is ruptured, the sulfides will be released into the tank and wipe it out. This stuff is very toxic (to the tank..not humans) in pretty small amounts.
Now...both of these "faults" have pretty easy solutions. They range from occationally replacing small amounts of the sand bed to making sure you dont get fish that will decimate your pods
The link below is to an article that details (in laymans terms) how the entire process works. Its not a long read, but is well worth it.
I hope that helped!