I had my tank start to leak a few months back, it cracked around the spigot upon which it sits. It split on the seam where it is pinched during the molding operation. At the pinch line the two sides of the plastic which are semi molten at the time are squeezed together and they set as if welded. That is the idea but the join is the weak spot and sometimes it leaks later.
If you know what you are doing you can weld the seam, but generally you should replace the tank and save the hassle. My one also cracked away from the seam, I think I could fix it but I have a new tank.

When you replace it you should add a plate with a hole for the spigot and put some silicone between the two faces to support the tank over a bigger surface to spread the load. I did that when I had the tank out to clean it inside but it still cracked. Good idea though despite the fact it cracked, maybe without the plate it would hsve cracked earlier. This is all explained in lugnuts article.

The area the tank sits gets a lot of crap in it, clean it out to stop it acting as a damp spot encouraging rust. I cleaned out mine and sprayed a rust proofing product in there, something like waxoil. The hose from the drain in the cowel is in there and l intend to make the hose longer and drain onto the road not inside the fender. The hose I add will have the inside diameter the same as the outside diameter of the drain so no potential catch point is formed.