You have to pull the engine and hyd pump frame to change or repair fuel tank because it's underneath the engine. The tank is polyethylene (plastic), common problem is small pebbles get lodged between belly of machine and the fuel tank. Over time vibration causes pebbles/stones to wear a hole in the tank. I've also seen hyd hoses on top side of tank rub holes in the tank from running vibration. It's about a 10 to 14 hr job to pull engine and replace tank for an experienced wrench.
Make sure your leak isn't coming from something on the topside before you commit to replacing tank, even though it's not easy to check fuel hoses and tank condition on top side, just can't get to nothing on those things.