Are you sure it's the fuel tank? Is there any additional detail you can give us, for example, does any warning light turn on? It might be worth checking these: http://www.startmycar.com/ford/focus/p…
I had the same issue. My car is currently in the shop and I have been told it needs a new purge vavle, fuel pump, and fuel tank for a total of $2400. My extended warranty will only pay for half as they state Ford knows there is an issue but they will not cover it under warranty.