[A year or so ago I bought a 5lb kit of Magic-Sculpt for a project, and I keep finding uses for it. They claim it never hardens up, unless mixed, and mine hasn't yet.
The 5lb kit would be enough to last a lifetime for most people, and this is handy stuff. It would probably be perfect for what you are trying to do. On their FAQ page it says it is aquarium safe (with an extra day or two to cure to be extra-safe).
It's basically two-part resin that takes a little while to cure, and is soft and hand-workable like clay. I can vouch that it cures rock-hard and can't see how it would ever leak.
Unfortunately you wouldn't find any of it locally, and would have to order it from their site. Here's a link to their FAQ.
http://www.magicsculp.com/faq's.htm
The stuff does get a little tacky when handling it, but you can dip it in water, or dip your fingers in water to alleviate that. (just do not wipe your hands on any cloth that you care about, voice of experience here
)
Plumber's Putty would probably work just as well from one of the local hardware stores, but would be more of a mess to handle (I think) and would cure faster, so you would have less time to smooth it out and shape it. It would also cost more per ounce, but would come in smaller amounts.
Anyway, I thought this stuff might be good for what you are trying to do, I keep finding little things to fix with it, and it seems like a pretty well-kept secret because it's not in stores.]
