My tank is 75gl and I have rocks in there. The only reason I choose to combine them because, on live aquaria, they say that Chromis encourage the other shy fish to swim around, these two are peaceful and compatible with each other.
I also have a blue Tang, and after we moved she has become very shy and stay hidden. One of the member above suggested that Chromis will school if they sense a threat. So the blue tang can be a threat to them.
Any suggestions for any other peaceful fish which schools better, but not the Cardinals, they are very slow and the Tang will finish all the food before they can get to it.