Acrylic cracks

this is what i use and it works miracles

sealed plenty of tanks.
drys quickly easy to use

get it at tap platics or any hardware store

looks like pretty thin plexi for a tank that size.

Weld on $16 will make a pretty sizeable mess of that cracked area if you just try to fill the crack with it. I'd suggest carefully (but quickly!) painting some weld on $16 onto a patch peice that completely covers the crack from the inside, and then slapping it on there, so you've effectively laminated over the crack. A repair like that will take at least a week to properly cure though.

There other ways to fix that too, depending on how much time/money you want to spend. You won't have something unusable if you pick it up though!