Definitely silicone. I always use it when I build drainage tables out of OSB, 2x4s, and pond liner to make sure the E&F fittings don't leak. Never had any leakage problems. Epoxy would probably work just as well, but as Freezer said, it is much more permanent.
I use a silicone that is use for Lexan plexiglass. Its called "Lexan Glue" and sticks to hydro equipment as strong as epoxy yet remains flexible and can be scraped off if need be. I've use a shit load below the water line and never had plant problems from toxicity and never losses its bond when submerged for months.
Lexan Glue / Silicone. Found in the window repair section of your hardware store. Comes in caulking gun or squeeze tube sizes. $5.00 to $8.00.
You can thank me later !! :smile: