yes you can use it in aquariums under the water.. it just has to say 100% silicone
on the bottle and your fine.. i used ge silicone from home depot when i built my sump.. you dont not have to use that aquarium safe silicone if you dont want to ... it just cost more.. The GE brand works perfectly fine. just allow a couple days to cure..
When i built my sump i searched all over the Internet to find out if GE silicone or any silicone that was 100% silicone was ok..And of course it was..
You are right about the mold inhibitor.. It cannot be in there at all...
My tank is now running and has fish in it.. Everything is fine..And i saved a few Bucks.. Just use GE!!
Ohh and here is why it has the weird wording about use in aquarium and below water line
GE 1 had "aquarium safe" but then they starting supplying All Glass with silicone, part of the agreement was that GE would print "not for aquariums" on the label. Also the label as a whole has less info now then before. A lot of times the "aquarium" branded ones simply cost more because they know they can sell it for more to niche market.