Do you have any idea what the tank is made of? Type plastic.
Several manufactures make a plastic repair kit which includes the heater iron and air pressure, and the right kind of plastic stick to match the tank plastic.
The process is similar to brazing, in that you melt the plastic rod and cause it to flow where you want it to go.
Plastic Welding Kit
50 Piece Plastic Welding Rods
If you can match up the plastic rod to your tank, you should be able to weld it.