If you got to your LFS, or PetSmart / Petco, they sell aquarium safe silicone. This is a tiny project, and a $5-$7 tube will do the job.

I mean, sure, you could go to Home Depot and find a tube for $4 that might not have anti-mold toxins in it, but since you are already saving on the project by building it yourself, I don't see why you can't just buy the known safe stuff.