Well aquarium silicone is fish safe, and will bond the pvc pipe.
My friend use "Marine goop" This stuff is amazing. He used it to build his protien skimmer. He rinsed and has no ill effects from it.
Regular silicone that has no anti mold agents in it will also work. If you read a tube of aquarium silicone, you can always compare ingrediants to the cheaper ones at your local hardware store.