If there are any traces left visible, remove. Then I would use painter's tape to expose where you want to silicone but a few mm wider than originally, so the old silicone is covered up completely with the new silicone. Slather on the aquarium safe new silicone on thick (its a sump so it doesn't matter how it looks) and I would think the sump will be fine. A thick layer of the new silicone should prevent any traces from leaching into the sump (although after a thorough rinse) you could test it out by filling with saltwater and testing it with a frag or something before installing it on your system.
Anthony
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If you see it, can take care of it, better get it or put it on hold. Otherwise, it'll be gone & you'll regret it!
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