I have a 4" plus deep sand bed in my 75gal reef. I have noticed a few things that differ from my tanks in the past. First off I put the sand in when I first setup my tank and i used 100lbs of Caribsea live sand and I picked up 75lbs of live sand from Premiumaquatics.com. My tank cycled in just over a week. The Deep sand bed is a great place for bacteria, pods
, and such that will help to provide stubility to your system. The most important thing that I have noticed is that I have reading of 0 Phosphates and 0 Nitrates in my tank (6 weeks old) and I have yet to do a significant water change.
here is a video that provides a bit of info about deep sand beds (he has a few others now, to)
How To Setup A Saltwater Aquarium Deep Sand Bed - YouTube
Hope this helps.
75 Gal Reef Build