Aquarium safe silicone

If it isn't 100% silicone I wouldn't use it in my tank.  Not worth trying to save $2 to have chemicals leaching into the water.

I have used aquarium silicone in the past from all glass aquarium that runs around 7 dollars a tube but GE 1 window and door. It works great and works just the same for 4-5 dollars at the local hardware store. Make sure to use the window and door version and use it in a open area with good air flow. Great for DIY backgrounds