Doing water changes is your best bet, until your nitrifying bacteria can catch up with your ammonia. Just keep it very low. Ammonia is more toxic than nitrite and way more than nitrate. I believe there are supplements you can use to help your bacteria multiply, but am not a expert at that so i cant give you feedback on that.
Even though your fish is a juvy does not mean it needs less space than an adult tang. I had a two inch yellow tang in my 30 gallon. It started getting ich and fin rot. I traded him in at my lfs
because its not fair he has to go through it because i wanted a good looking fish. I had a blue tang i traded as well.
I like you added 4 fish at once to causing a huge bioload. Consistent water changes, daily water tests help me beat my ammonia spike. I just have issue with nitrates atm but thats a different story lol