Thanks for the update Bill-- I can't recommend RO/DI enough-- I was using RO only up until last summer, switching to RO/DI made a big difference in my tank.
I've been following this thread for a while so I think I can answer some of the recent questions-- anyone feel free to correct me...
@ Rusty-- I don't think this is the proper section to ask about a mangrove refugium-- if Anthony does check this thread he would likely be able to answer that.
He (Anthony) did write an article that someone linked to on the previous page of this thread that you might find helpful...
http://www.reefkeeping.com/issues/2004-12/ac/feature/index.php
@KH971 -- I don't think it will be too detrimental for the RDSB to get light-- I think the idea is to NOT make it a live sand bed-- absence of light should aid in critters not taking up residence-- as well as any nuisance algaes that might start growing and providing habitat... I don't believe it is absolutely necessary for it to be dark, but I believe that was the idea of covering it.. For sand depth, the deeper the better IMO. For sand grain size... you want fine sand-- oolitic/sugar grade... I believe Anthony recommended 0.2 to 0.5 mm grain size.
@Kramer-- The beauty of this method is its portability-- easy to remove and replace. I'm not sure of the necessity to replace it after a certain period of time-- what exactly is supposed to be occuring that would necessitate it??? Remember this is not designed to be a Live sand bed... the issues or a DSB (in tank) crashing is a result of too much detritus accumulating and/or accompanied by the loss of biodiversity of detrivores in the sand bed. Since the RDSB is purely bacterial, the possibility of a crash is likely less...
But irregardless, this is what I would do to replace it (to minimize any "shock")... I would start a 2nd RDSB--in series before or after the existing one... let it go for a month or two to let it start taking up some of the "load" from the other one. THen to prevent any big shock you could either start removing sand from the existing one (about 1/5 of the volume per week or so)..... alternately just remove it entirely--I don't think you would see a serious nitrate spike-- but if you do, it should be short lived.
HTH...
I'm by far not an expert so anyone please correct me if I put out any bad/incorrect info... some of this is my opinion based on what I've read here and other places..