Don't add crushed coral to the top of the sand. I would only put in an inch or so of sand to start. Seems to be quite a little controversy around DSB's and OTS (old tank syndrome) these days. To be on the safe side, I would hold off on putting in the full 4" of sand. Putting it in the refugium instead may be prudent so it can be removed/changed in a few years if DSB's are shown to be related to OTS. I'm not convinced yet but I would play it safe in a new setup.
Not so much trace elements building up (actually heavy metals according to Dr Controversial, er I mean Ron) but a build up of toxic gasses in the nether regions of the DSB which, after a period of several years, mysteriously make themselves available to the water column and poison the tank. Again, I am not convinced that all DSB's lead directly to OTS with only anecdotal "evidence" of stories of other people's tank crashes. I haven't read of any scientific studies other than Dr. Ron's study on the build up of heavy metals which after several years, supposedly, renders your whole tank useless (sand , LR and tank) until cleaned with muratic acid. I have read about OTS but have not experienced it directly (yet). What opponents to DSB's claim is OTS will not occur in a tank with a bare bottom. I don't think these folks have run a bare bottom for over 5 years to be able to tell. There may very well be an unknown mechanism which causes a reef tank to crash after a half dozen years. Dr. Ron calls it heavy metal, others blame toxic gas in a DSB. Dunno...
How long were these systems set up? Unless they were running for longer than 5 or 6 years then not having a problem (with tank crashing) is the norm. It is in older setups (6+ yrs) that the problem seems to manifest itself. Although I have kept reef tanks for a dozen years, the longest one tank has been set up is 4 (nearly 5) years. I can say I have had no problems with a DSB. It is in the following years that some would say I will have problems.
