[kolokefalo]

hrmmm this is a toughie but Ill try to give a read on a couple possible culprits. I am assuming when you fished the dead ones out you didnt notice any current in the aquarium so I would rule out a malfunctioning power head releasing charge into the water.

Was the live rock purchased "uncured" or shipped via the mail? Although you are showing Nitrite is perfect I am assuming the die off from the rock flooded the tank with ammonia.

I am not sure why lfs told you you needed damsels in a tank with reef rock. The die off from the rock and the curing will cycle the tank well without the addition of damsels.

A few more details will bee helpful:

ammonia reading?
Live rock and sand from where?
The symtoms you are describing of the fish just losing color and falling to the bottom is usually a acclimation problem. (although you mention it happened on the 3rd day which is vexing)
Was the Wet/Dry new or cleaned before installation?

One ver remote possibility was lack of oxygen. A powerful wet dry with live rock as well can cause low oxygen levels. While this isnt eough to kill fish and is easily remedied by a little surface turbulence possibly with the ammonia spiking and the lack of oxygen it kind of created a perfect storm kind of thing. However, that is just spitballing.