I'm just returning to the hobby after a very long hiatus, but I had a very happy and very successful 5-year-old 55-gallon and a 6.5-year-old 120-gallon display tank with deep sand beds, from 1987-98 (started saltwater in 1986, after almost 15 years of FW.) The reef world has evolved drastically since then, mostly for good, but I disagree with the move away from deep sand beds.
I'm a huge proponent of keeping as diverse an ecosystem in my tank as possible, including a DSB, live rock, microalgae, and a purposeful fish and coral selection. I'm more conscious of environmental impacts now, so I wouldn't pick up Fiji live rock at the airport, even if that were possible. The cultured live rock they're making in the ocean in Florida looks pretty awesome anyhow. IMO, you won't ever have as successful DSB without quality live rock with lots of critters in it.
My DSBs always had 3-4 inches of oolite with about 2 inches of aragonite on top. I had already been 7 years into saltwater, so I had the patience (and bad memories) to let the 120 tank DSB establish itself with an extended 8-week, 4-stage nitrogen cycle until the bed had a good anaerobic bacteria layer that was converting nitrates to nitrogen gas before I began adding livestock. That's excepting the sand crew, which (excluding live rock hitchhikers) I added at 4 weeks. I've always used lab-grade ammonia to cycle tanks, cruelty-free. That's easier to deal with now that Dr. Tim's is around; I used to have to go to a chemical distributor.
When I started my 120, I had another DSB in my old 55g converted aquarium-turned-sump/refugium (kind of primitive by today's standards and homemade, just one glued-in divider with my skimmer, 2 heaters and a return pump on one side (1/3) and the input and DSB (2/3) on the other) that only had (after nitrate showed) a diverse macroalgae farm of gracilaria, chaeto, several caulerpa types, and dragon's breath in it as pod habitat to feed my future blennies and mandarin, as well as for overall water and tank health. I don't know what the big variety of macroalgae did, only that I wanted whatever it contributed to the ocean to be in my ecosystem too, and my tank looked awesome and was remarkably stable, so I guess it worked. The macroalgae barely grew after the DSB fully cycled, and I rarely clipped it, maybe every two months, indicating that the DSB was doing its thing and exporting converted nitrates as nitrogen gas.
I landscaped the display with calcified macroalgae about 4 weeks into the cycle, and began light kalkwasser dosing, and used live phytoplankton and daily light feedings to encourage pods, critters, and my live rock to establish a very strong ecosystem. By the time I added livestock, pods were everywhere.
After around 8-9 weeks, I added a BTA and a couple of percula to start, then added a coral or fish every week until the tank was populated. I added bacteria for a couple of days every time I added livestock, to negate the added load until the system adjusted, which wasn't long, maybe 3 days.
My matured system was heavily-fed and nutrient-heavy and all I can say is that a DSB provided nitrate export without creating a giant glob of macroalgae collecting garbage in my refugium. In fact, my refugium was sparkling clean compared to most. Because I had enormous sand crews, including bristle worms, nassarius snails, conchs, brittle stars, a couple of sand-sifting gobies, plus whatever the live rock brought in, there was never any detritus, and my 120 was 6 1/2 years old when I tore it down. DSBs never once let me down, and the sand bed was every bit as healthy at 6 years, as it was at 6 months IMO.
With the exception of chasing a mantis shrimp that snuck in as a baby on my rocks, I never, ever disturbed my sand bed. When I did disturb it chasing that ****ing mantis around, it released absolutely zero waste into my tank and was as clean-looking on the surface (after cycling) as it was fresh-washed. It was so full of life, it had its own entertainment value and drama; the surface erupted with critters at feeding time. It was cool af.
It's my opinion (take it or not) that failed DSBs are due to impatience or lack of biological completeness. It's extremely important to allow it to become fully established undisturbed and to not vacuum your bed, ever, under any circumstances; a proper DSB cleanup crew does that for you. Vacuuming a DSB draws water through the anaerobic layer deep in the bed and the oxygen kills the anaerobic bacteria, disrupting it for weeks. You may have to experience a few weeks of serious ugly but resist vacuuming diatoms and stuff like that; they're natural and there for a reason. Rely wholly on your crew to take care of sand maintenance.
I think that tanks are more stable and have better overall health with a DSB, but mine's just one opinion. I have no desire for a bare-bottomed tank, ever. I'm setting up a small, 20g to experiment and play with (today!), and you bet it's gonna have a deep sand bed. I think millions of years of evolution trumps everything, but I'm obviously just an old-school old man.
