Thier have been so many changes and modification to a DSB that is pretty tough to figure out exactly what the concept is these days. But I will give you the basic premiss on how it works.
The soncept is to use the finer grade sand, the finer the grade of sand the more surface area thier is for bacteria. Also the finer the more well it packs and thus makes the lower section of it anaeobic (lack of oxygen). But personally the sand you have should be fine. The way the bed works is that in the upper zone that is well oxygenated (areobic) you will promote the population of nitrifing bacteria. these are the bacteria that process ammonia to nitrite and then to nitrate. This area will also have populations of lerger life forms such as snails, worms, pods and other simular type bugs. The role they play for the most part is to stir the sand in this zone allowing it to stay oxygenated. They also process food and waste particles to smaller peices. Everything below this zone is called the anaerobic zone, it is void of oxygen for the most part and accomedates a different type of bacteria that processes nitrate, from nitrate to nitrogen gas, which is then off gassed. A big player in this game is a type of worm. It travels between the areobbic zone and then holds it breathe for lon periods as it travels down to the anaerobic zone. this traveling brings the nitrates from the aerobic zone to the anaerobic zone bacteria, this traveling also allows for the migration of the gas being offed. Problems that can occur here is with the lack of these worms, thier is no transmission between the two zones. Also if thier is the presence of ammonia, it will not allow or denitrification. This is the basic concept, the cons are basically two fold. The process outlined above work only for products that are nitrogen based, so if you are putting stuff in the tank that is not nitrogen based it is not going anywhere, so it sinks. that along with all the things that shed and die in the bed. this leads to the eventual increasing of the ananerobic zone and the decrease of the areobic zone. so eventually the bed will cease to function as a filtration even for nitrogen. the second problem is that the DSB has no ability to deal with Phosphates. This will lead to a Phosphate cycle between algea and critter. The sand you use to make up your dsb is saturated with phosphates prior to even coming out of the bag. But it is locked up and wont pose an immediate problem. but what happens is that with the formation of an anaerobic zone, the PH in those areas will lower, as it lowers it frees up all the P that was once bound into the sand, this P will raise and be taken in by bacteria, resulting in the rapid rise in thier population. Once the food source begins to be exhausted the bacteria begin to die off, as this happens algae (usually hair or cyano) begin to take advantage of the new food source (sound familar????) thus it will begin to bloom. again once the food runs out the alge will begin to die off as it does the bacteria once again jump onto the rtting algae and feed, thier populations grow and the cycle begins all over again. Now if you add to this all the P we put into our tanks via feeding, additives, waste and so on this P cycle gets bigger and bigger unitl its out of hand. It is this that usually makes reefers tear down thier tanks and not the first problem.
You can add all the snails and crabs and worms you want but the problem is that what ever they eat or take up does not export from the tank, it stays their until they die and then once again joins in the P cycle. the same applies for algea type scrubbers or calurpa refugiums. yea they will uptake P and nitrogen and you can havest them (thus exporting) but they do so only by removing what ever Inorganic Phosphate is in the water column, they dont touch the P that is in the sand and they cannot compete with cyano or hair algae when it comes to uptake. Dr Ron and other experts state that the concept is that the harvesting od cyano and hair from the DSB surface is the method for exporting that P, but I personally find harvesting cyano to be a bit unattractive.
OK on the BB concept, the idea is to export the food/waste/detritus prior to it being reduced (rotting) thus skipping all the processes listed above (pros and cons). als we have LR in our tanks that do everything that sand does but at a samller rate. the difference in the LR is that it has no bottom, so through bacterial action (tugor) the detritus is actually pushed out of the rock, thus not allowing for the eventual filling up.
Sorry for the long post, but hopefully it gives you and Idea of all your alturnatives
Mike
(any ideas why this would be way up, I tested 3 times to be sure it wasn't human error) , Alk == 5.92 DKH, Calcium has dropped to ~305 
