What are your tank parameters, how often do you feed? What filtration are you using?
Do you have another quartine tank for the clownfish if you needed it for a hospital tank? Your going to need to treat that clownfish with copper in a Med Tank.
If you have a LFS that sells 2.5 gallon tanks, buy one of those, a filter and heater for it too. Buy this product called:
Aquarium Fish Medication: SeaCure Copper Treatment
Check your LFS to see if they have any copper treatments there so you don't have to order one online. You can use the same water from your tank for the small hospital tank.
Do this now before you lose him!!!