It took me a few minutes but I found this in an old thread.
"I was told not to use the GE silicone II because it had mold protection stuff in it, the chemicals for the mold protection will not allow algae to grow, and it will be harmful to the fish, hopefully with a tank that large it will dilute all the chemicals."
There is a sticky at the top of the DIY section on Silicones that you might want to take a look at.