Hi,
I have a four foot tank that cracked due to being on unlevel ground.
I spent a $1000 on buying the tank and almost died thinking it was doomed.
All i did was re level the tank using a spirit level to make sure it was dead flat and used some aquaruim sealant from bunnings to seal the crack. I sealed both the inside and outside of the tank and used a putty knife to push the silicone deap into the crack. Once it dried i used a razor blade to scrape off the excess.
It has worked great so far for 3 months considering the water pressures in such a large tank.
I think the most important thing is, is to make sure the tank is dead level so all the pressure is distributed evenly,
Good luck.