You're insane man. That is one expensive and beautiful fish. And trust me saltwater is absolutely NOTHING like freshwater. It takes 10 years just to master saltwater haha. You're going to kill that tang and its a shame because she is beautiful and doesn't deserve a 25 gallon or 100liter tank. I have a 180 gallon/680 liters and I would be hesitant on a purple in my tank that's been established for almost 3 years. Please listen to everyone here. There is a lot of knowledge and you're torturing that fish. LFS should have never sold you that fish honestly and shame on them for it if you told them the size tank it was going in. For the fish's sake put him in an established tank and if your 200 gallon is freshwater I'd start converting now because it takes a long time on a tank that size to work out the kinks of a new build. No one is harping on you is just this was honestly a terrible move on you and you fish dealers part.