I'd say you have gotten perfectly sound advice so far. Your first post indicated that you intended to paint and seal the deco. Our advice was to do just that, and do it well, and there should be no problems. If you seal it carefully with many thin coats it should be basically impossible to "miss a spot".
Is bondo toxic? The MSDS contains toxicity information on the raw paste, fumes and on the dust created by sanding, but to ask what happens after it cures and is placed into potable water might be beyond the scope of this forum. The proprietary resin it's made out of is probably polyester, but the bulk is talc and metal powders. Since it is porous, I would expect it to slowly dissolve in the weak acid bath of a fish tank, and you might then get a toxic effect... does that make it toxic?
Just seal it. If it sinks, it'll work.