lowes and home depot both have aquarium safe silicon, its by DAP. petsmart and petco both carry the silicon as well, just ask for it, HD and lowes won't know what you are talking about, you have to look where all the glue and adhesives are. thats where i found it.
I have no idea how i had any left. I used quite a bit so i was sure i had it securely on the tank sides.