...sigh...first of all, you should technically QT ALL purchases...fish, corals...before adding them to your tank so that you are assured of not adding any parasites or diseases to your current fish/corals.
And pretty much nothing that you add should just be dumped in the tank, they should be acclimated to your tank.
They were living in someone elses water, lighting..etc before they came to you and before they go into your tank you should acclimate them to the temp of your water, and all of the other parameters including salinity.
Best to find out when you purchase something what conditions it came from..ask the seller.
This is important stuff Oriol..so I'm going to ask that you do a search on acclimation and I'll bet you'll find a bunch of stuff to read about it.
As far as the Hippo, it's always best to research any fish or coral you buy prior to purchase. If you google Hippo tang, you'll find that it is not suggested to keep one of these in a tank smaller than 100g's..they get big and all tangs are poop machines, which means he is going to foul your water on a constant basis.
Fish do NOT grow to the size of the tank we put them in..they may grow smaller than the size you would commonly find them to attain, but that is ONLY BECAUSE WE HAVE STUNTED THEIR GROWTH..(oh, did I yell that?...)..so it will live an unhealthy life, never thrive and that shoots down the reason we are all here which is to learn and provide the BEST environment for the fish & corals in our care.
She now kicks the soapbox to the side...
I know you said you'd either upgrade to a larger tank or give him away before he grows up and I hope you still are sticking with that plan. For now, let's get him healthy again
You're doing great Oriol and moving in the right direction