There are two things you need to be very careful of, that the silicone needs to be underwater proof and that it is safe for aquarium use (very important).
A pool grade silicone would meet the criteria of underwater use, but I'm not sure about aquarium use. Try asking at your local aquarium supply house what your looking for is like the product at this link:
Ace - Ace Tree: Paint: Caulk & Sealants: Pure Silicone Caulk: Dap® Aquarium Silicone Sealant (00688)