Alot of great information gathered here. I concur on most of what is said about DSB's, and I DO agree there is a Finite end to them, but in most cases that Finite end is going to be Way longer than the tank was set up anyways. At least if you're a responsible reef-keeper like me
I still advocate, and use, DSB's in my systems. I have a 2 yr old bed 3"-4" in the 135 reef at the shop, a 5 yr old 4" dsb in my 75 gallon main tank (at home), a 9" DSB in the attached 55gal refugium (6 months old), and a 2" dsb (using silica sand no less) in my 10 gal nano w/ dwarf lion.
All of these beds are maintained on a weekly basis by siphoning free-floating crud from the bed's surface. I kick up a very fine layer from the top, but otherwise leave the bed untouched. I also have an overflow sponge in place, rinsed bi-weekly, and a filter sock on the overflow from the tank to catch any other lg debris.
My main system at home (the 75 w/ 55 refugium) has extremely fast-flowing water (Over 30X's/hour) and my bed does not stir like I hear so many people having problems with. However, I also went with a more diverse bed, including several grades of substrate, from a majority being fine sand to a minority of crushed coral. The top is a more diverse region and I think helps to keep the bed from being blown away.
I have to mention the fact that the 5yr DSB at home was upturned once about 6 months ago, when the refugium was added. My old tank broke a seal on the bottom and had to be replaced. I tore the bed out and drained off all the brown "sludge" that settled to the top of the buckets the sand was put into. I then re-used the sand and added an additional 40# of Florida Live sand from the Gulf. Otherwise, the bed has been in operation "as-is" since it went into the tank, with the exception of the maintenance tasks outlined above. I lost one staghorn during that whole tear-down and set-up, and had a few corals bleach for a few weeks before re-gaining color.
I think that rinsing out those buckets made a huge difference in the lifetime of my DSB, and it showed me that it's not impossible to do this maintenance task if needed every few years. I'm not saying that it's something I'd enjoy, but it did seem to give more life to my DSB. Shortly before the old tank popped the seal, the same dsb, undisturbed, was starting to have red slime issues that I could not overcome. After the new tank was in place, the red slime has not been seen, although red planaria flatworms have infested my tank in it's absence
The refugium has not shown much in the way of sand diversity in the deeper layers. It has certainly taken off in the way of life, with tons of bugs and worms, not too mention lots of chitons and other asstd tiny snails. The macros have all taken off as well, but nothing IN the sand layers to speak of.
One last thing that I think helps my system to work well is the fact of the bare-bottom initial sump chamber. This is where the filter sock collects water from the tank, and gets siphoned as needed. Because of the waterflow in my system, the detritus gets carried out of the main tank (hard for things to settle in a tsunami
) and deposited into the sponge, filter sock, or first sump chamber. Whatever manages to get past that is THEN deposited to the fuge for waste management.
As you can see, I have lots of methods for nutrient export in-line to handle most of the organics. Doing this isn't going to make my DSB invincible, but it certainly helps in giving it longevity.