Aquarium specific silicone is going to be expensive. You can use GE Silicone 1 from HD/Lowes, or you could use Momentive RTV 108. I have also used Permatex RTV you can pick up at Auto Zone. RTV has a higher tensile strength and is used for building tanks, GE I will work, but it's not as strong. Grainger may have some RTV in stock.