The common nickname for all tangs is "Ick magnet."
Trust me. Is has ick. It's a tang and you didn't quarantine it. It has ick, simple as that.
The tricky part is...WHICH ick does it have? There are many different saltwater "icks' out there. All of them are very bad. Very, very, bad.
You might have gotten lucky so far with all those other fish, but now everything is different. If you don't take immediate steps to get ahold of this situation, it is going to get nasty. I hate to have to scare you like that, but it really is that serious. An infected tang in a 75 gallon tank already full of other fish is the match to start a wildfire, and in a tank like that, ick does indeed spread like wildfire.
There are several options available for treatment, some of them almost easy, some a royal pain. There is a new thing on the market recently called "Herbtana" or somesuch which has a sister product by the same company. I don't remember which one it is that you might want, but one of them is supposed to be easy, safe, and effective. I haven't tried it, though. I offe it as a possibility since you're 11 and probably don't have the extra tanks and money it would take to try something else.
Good luck. You'll need it.
By the way, please ask your well-meaning relatives to not buy you any more fish, since it's important to know about saltwater fish before you buy them. Powder Blues are a notoriously difficult species to keep for very long, which you could have known if you looked it up first, which you might have done if it were you shopping for fish.