What I mean is, a full grown tang is a pretty large fish, and even in say- a 125, it's not going to fit very well. Imagine an 8" long tang with a round body in a 6' tank. I mean that there are not many people that have tanks in their homes that can house a full grown tang. Eventually most of those folks will have to find a larger home for it. A friend, a public aquarium, a larger tank...even tossing it back in the ocean if you live close enough to the ocean.
I wouldn't put too many tangs in a 75. One is enough IMO
. I have a 4' tank as well. It's 24" front to back though, and I have 2 tangs right now that are pretty small. They won't be with me forever unless I substantially upgrade.