Ur bracing crack bcause u don't have a centre brace. If u want it to be save , draw out 70% of ur tank water , cut off the whole strip of the crack bracing and replace it with a new one and add a 6 to 7 inch centre brace and let the silicon to dry min 2 days before u fill ur tank full with water . It going to be tough alot of job to be done if .not it like a time bomb. Good luck .