The more I read on this the more confused I get...
How on earth do you have an 8 year old blue tang that is only 4 inches long? Either the tang is severely malnourished or it isn't 4 inches long. My blue tang was over 4 inches long at under a year old and it was the size of a nickle when I got it. I don't see how it could be very active in this tank, it has to be basically break dancing spinning around on it's head, getting dizzy. Regardless if the tank is round or not, that tank is way to small. What is the diameter?
I'm glad to see you care about the fish and nursed it back to health, but I think it is in the fish's best interest you trade it in, sell it off, give it away, or get a much much larger tank. You could look at getting a blue tail damsel, they look similar to blue hippos and are much more suited for that size tank.
The comment about it being proven time and time again they can be survived and are tank bread is just wrong, they are not tank bread and it has not been proven they can thrive in a small tank. Typically somebody with a two month old system will exclaim they have successfully kept the tang in that small of a tank, then a few months later ask what is wrong with their tang or why are their fish dieing. It is almost always a repeated cycle. If you look at people saying they can be kept in small systems, majority of them are relatively new to the hobby...anybody with a decent amount of experience (over 5 years) would recommend against keeping a hippo in anything less then a 120 and most wouldn't recommend less then a 180. A tang can SURVIVE in a tank that small for a limited time, but not thrive, and is a ticking time bomb. The fish will eventually break down and cause problems. Also the shear feeding requirements and waste production of an adult hippo make it next to impossible to maintain any sort of solid water parameters in a 25 gallon tank. They eat a lot, and poop more than any other fish, and resperate like crazy.
Good luck.