lovethesalt,
if your live rock has been in there for at least a couple of months or longer, your sand bed is now a live sand bed. the critters will migrate from the rock to the sand. NOTE! This is not to say that the sand bed would now support a mandarin, or that a livesand activator, or detrivore kit would not be a good idea ... it's just to say what you put in as a dead sand bed would now have life in it.
Personal Experience
I have 2 Mandarin Dragonnettes (1 male and 1 female)in my 180. I have 2 - 4 in of sand and about 250 - 275 lbs of live rock. They are both still doing great. Ive had the male for 1.5 years, the female since Jan, 2000.
I have observed them "dancing" together right before the lights go out. It is beautiful! The tank is set up as a "spur and groove" with rock on either side. Normally they stay apart from each other... one in one rock pile the other in the other, they will switch sides, but normally stay apart. But, from time to time they will swim to each other, near the bottom to mid-depth, meet and then swim in a circular motion, swirling to the top of the tank. They will then swim to mid or bottom and repeat. It is really nice to watch, although you feel somewhat like a voyeur. I've not seen any eggs, or sperm release, just this dance.
It is also of interest to note that the male does not always have his front most dorsal fin upright. It is a red color at it's base.
Personal reccomendations
At least 100 lbs of LR per fish, and at least 75 lbs of LS per fish. Perhaps a bare bottom if you have a good growth of calurpa for breeding the 'pods.
These two beautiful creatures are on my website under the livestock heading.
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[This message has been edited by wayne (edited 08-14-2000).]