It was fortunately a garage sale find.You said it's new, so I'd return it for one that isn't busted. Don't try and fix that, especially with something you got new.
The crack is in the bottom left corner of the aquarium.Is it in the side or base?
I’ll do this, then make it into a paludarium.If you can't return it, I have siliconed cracks of that size on small tanks successfully, but it is not ideal. The one consideration is that cracks that terminate mid glass plate will typically continue till they get to the edge and you don't know where they will go till they get there, so I will slightly push on them till they run to an edge then silicone them. There is not a lot of pressure on a 5 gallon tank, but 5 gallons of water is a lot of water if released in your home.