John, don't mind Uncle, he often posts conflicting information (at least in these past few days he has) to better his position. Frankly the whole tone of his posts are rather irritating already, like his opinion is the only right one.
I personally have used GE Silicone 1 - Windows & Doors with no issues thus far, this came at great recommendation from 3 friends and the LFS shop owner who have been keeping reefs for many many many years.
I would trust their judgement over someone on the internet any day
but don't take my word for it. :fish1:
Being in the construction business I can assure you that Silicone 1's would be safe, as long as they are allowed to properly cure, and by cure I don't mean what it says on the bottle, I mean a week at the minimum just to be safe. What ends up being the deadly factor with these Silicones is the off gassing into your tank. Ever wonder why the smell burns your nostrils when working with it? Can you imagine the fish having to breath that in.
Companies like GE don't endorse their products for aquarium use because they don't want to be held accountable for any failures or liability claims. Just make sure it doesn't have the mildew inhibitor in it and you'll be fine (GE II, etc).