1000% agree with Sniper. A Hippo in a 20 is a really, really bad idea. They need space to swim. Minimum tank recommendations are not for fully grown fish or they would not be listed as "minimum". Tangs stress easily and get stuff like Ich easily. They are great fish for a proper sized tank, but this is not even close. Any
LFS that gives that as a solution is looking to take your money.