SILICONE SEALER that is manufactured for construction will sometimes contain ARSENIC. The arsenic is used for prevention of mold and mildew in bathrooms.
Aquarium sealer manufacturers do not include arsenic.
I also want to add that if you have an old aquarium (and I do mean old, 20+ years), chances are that they are built using silicone that also contains substances that are toxic. Although that's primarily a problem if you are keeping rodents in them.
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Hopefully your plans will actually go somewhere. I'm still waiting for my husband to start making the insert he was "inspired" to do some months back.........