It's probably sick
It might have gotten exposed to high levels of nitrite. So let's see, it is now Thursday, so you've had the fish for a week. That is quite long to go without food. It doesn't take that long to acclimate a fish to a new tank.
If it were me, and again I am pro-active, this is what I would do. I would use a wide-spectrum anti-parasitic treatment and follow the course of the entire treatment. Monitor the fish carefully during treatment and see if you can see any signs of improvement. A good candidate might be one of those tank buddies (Jungle Lab?) products indicated for parasites. They work really well. This should take care of your more common parasites, if the fish is indeed infected. If that doesn't work, change % of the water, filter through carbon, and then add a wide-spectrum antibacterial. Do the same as above. If you suspect that the reason why the fish is not eating is due to exposure to high levels of nitrite, I would use methylene blue instead of the above remedies. None of these medications will hurt the fish if you follow the directions. But again, I am pro-active, as opposed to conservative. You might have a different perspective on things.