Sorry, I did not communicate my thoughts clearly. I was talking about watering the terrarium. What I meant was in the completed terrarium (once the silicone cured) with the leachie living in it, you would spray water on the terrarium occasionally for humidity and drinking (although mine drink from a bowl, so mostly for humidity). The water sprayed on the glass and tank would only have a short contact time with the silicone, so the arsenic would leach out of the silicone at only a minimal amount. You need (and should) only use as much silicone as needed to get the job done.
Basically, commercial grade silicone has arsenic in it for antifungal reasons and this arsenic can/will leach out when in contact with water. Since a terrarium (without free standing water such as a pool) would have little water contact with the silicone, the arsenic only leaches at a very slow rate. This rate is probably (but not guaranteed) to be non-lethal to the inhabitants. If there is a waterfall or pool that contacts the silicone the leaching of arsneic (or other toxins used to kill mold/fungus) is a real issue. If you use a food/aquarium grade the leaching of arsenic/toxins is a non-issue. I hope I cleared it up.
Regards,
John
