This may not be news to some, but GE has started adding a mold inhibiting agent in their GE brand Silicone #1. This unfortunately means the product is no longer safe to use for new background projects.
The good news is Marineland makes an aquarium safe silicone you can buy in the same size tubes for your caulking gun. Here's one place to get it: http://www.drsfostersmith.com/product/p ... catid=3963