If you're looking for aquarium-safe Silicone, try Home Hardware (I went to the yonge/St. Clair, Toronto location). They've got 84mL tubes of clear silicone caulking there for $3.99. I believe the brand name is Weather Shield, white tube with big, baby-blue "Silicone" written on the tube. It states on the tube that it is safe for aquariums and I've used it with no ill effect.