Nerf supermaxx 5000 tank leaking. Solutions or at2k tank?

I recently purchased a nerf sm5k and after I pump it up, the tank slowly let's air back into the line for the pump until the tank is empty.

I wanted to try and fix it but don't even know where to begin. Is there anything I can do to fix it maybe without taking it apart? Not sure how the internals work on this blaster but maybe one of you guys know?

Also I'm interested in an aftermarket tank as well, like the at2k but I don't see anywhere to buy it. I did a lot of searching but couldn't find where to buy those tanks.

I would prefer not to get a tanks from another supermaxx blaster...

Thank you for your guys help in advance