Hmm, make sure the plastic is clean... of course it depends on what kind of plastic probably, I've bonded many plastic things with silicone. I've got a whole box of silicone from when I worked for the cable company (we would use it to seal up around were cable intered into the house) and as long as the plastic is clean and completely dry, it works fine.
FWIW I'm sure its a lot harder for a product to be approved for potable water systems than aquariums.