Replace it dude,cracks in AC's usually run longer than your eye will make out,it's hard to get the whole length.I did this,waited for it to dry,found out the crack went around a corner and continued!!!!
A hot glue gun will be fine. I use the clear glue rods in it and I've glued things to the tank bottom and just the other day used one to glue a suction cup to the bottom of my bubble collum. Nothing there to harm your fish.