Glass softens at 900f you could get it to fuse at like 1000 to 1200... commercial glasses are manufactured by fusing of sand, limestone, and soda at temperatures around 2552 °F to 2732 °F..
Remember, you need to cool your glass from 900 down to 100 at a rate no faster than 100 degrees per hour. thes is called annealing. Glass which has not been annealed properly will crack or even shatter when subjected to even a relatively small temperature change or shock.
if you cool it to 600 degrees then do a rapid cool (with cool forced air), then you get tempered glass (if you get the coolong rate right)
If you are not a gaffer, you shouldnt attempt to melt your aquarium to fix it, you will only stress the glass & cause problems.
