New tank. Cyanoacrylate glue (super glue) will glue difficult plastics really well, and you can get it super thin flowing in to the smallest tiniest cracks. Quite chemical resistant but it might end up slightly brittle compared to the OEM plastic in your tank.
I would recommend "ZAP glue", they have really good CA glue that don't get too brittle.
Be careful with it, it etches in to the plastic immediately so stains or excess glue don't go away. Try it on something else first.