You'll have to be more specific about where or how it is leaking. Sometimes a fitting can be replaced, or you can buy a plastic welding kit for $50 or less. That's like a hot melt glue gun, but it uses plastic sticks that match your tank material and fuses the crack with new material. Sometimes MarineTex will do, but getting anything to stick to poly is a gamble. Sometimes you can through-bolt a plate over the crack.
Sometimes it is simpler to just remove the tank and replace it.
It all depends on the details--which you haven't mentioned.