Most of the problem with glass is the green taint. With starphite, you can go with 3/4 inch without any difference in this color problem. The 50% increase in weight (from 900 lb to 1350 lb for a tank of your size) should not be an issue, since you going to need a cherrypicker or a team of people to do the moving and installation anyway.
I think the real question is what is the safe thickness of the glass.
If you use the www.garf.org tank calculator you will get 1/2" (assuming the tank is 30" tall and not 72" :lol: ). This would give a reasonable safety factor; based on how they recommend to brace the tank.
If you are bracing the tank differently you perhap should go up to 3/4" for the sides. However, you might want to consider a 3/4" for the bottom to help support the weight of the rocks and sand reguardless.
Going to 3/4" for the sides would NOT eliminate the need for a center brace. JMO
