Its actually 125 gal tank, and I could care less how big your tank is, I care about the tang. Its a fact that they shouldn't be in a tank that small. There are things I would like to have but my tank is too small so I don't keep them.
In your original post you did ask about adding more fish, so I was commenting on that, and adding that you're already over your limit on the tang. YOU are being SELFISH by keeping a fish in a tank that isn't big enough for it, but thats your deal I guess, and you're not the only one. Every single legitimate site and all the info shows that a 55gal is too small for a tang, its not just what I've heard. Someone who really cares about the animal simply wouldn't keep one in there...But again that's up to you.
For whatever reason, my guess is the shipping was hard on them, and because they were so small, I haven't been able to have success with the chromis. So instead of killing more fish I choose not to keep trying, but you can keep right on with your tang.
Contrary to how this post probably comes across I'm not trying to talk down to you, and its nothing personal, but you are on a fish/reef forum and WE DO CARE ABOUT THE ANIMALS, and we will comment when something isn't right. I wish you the best of success and I really hope that Tang is fine and happy and is for years to come, but with your size tank, you're fighting an up hill battle to keep that tang alive and well.