Unless you have immediate plants for a larger tank, return the Fox. This is not a small-tank fish.
Potentially large fish must have adequate growing space along the way, or they develop internal deformities and stunting. This weakens the immune system so they will be more susceptible to various health problems and almost certainly have an unhappy life and premature death.
My rule of thumb is, never buy a fish unless you now have the size of tank that the fish will require when adult. This can save a lot of grief for the aquarist, and hardship to the fish.
Fish buying on impulse from appearance is dangerous; I'm sure all of us did it when we started out, but many dead fish later we learned the error of that habit, so it is one to get out of now.