Be'n that you already have a pair of clown fish I advise at not get'n a third. Because the two that are pair'd up will team up and pick on the other one til it dies of a beat'n or stress. about the other fish and how they get along but I know tangs are a tough fish to keep. real finiky. I know you said you have live rock but you should have about 1.5 to 2 lbs of live rock per gallon. Now I know you said you have 100 gal
tank so it'll take quite a bit of rock but thats somet'n you can slowly chip away at. And as for what was wrong with the fish I'm not sure and sorry that it happened but in the long run I think someth'n bad would have went down with him and the other clowns.
Here are a few links to read that might help you out a bit. One lets you know which fish get along best with each other and the other kinda gives you a breakdown of things you should have for you're set up. Good luck.
Saltwater Fish Compatibility Guide
Stock list and tips for maintaining your SW tank.