I am almost psitive it is not the silicone. Was it 100% silicone or was it like the industrial grade. I do agree you dont want to put the mildew resistant stuff in there. THis is the stuff i used on my sump and it was absolutly fine. Its the same as the stuff they previously had which said aquarium safe. Theyt only put 'not for use on aquariums" because of the legal implications of when the tank blows out. Like i said ive used this and had my sump running for at least 6mos now and no proabs.
http://www.canreef.com/photopost/sho....php?photo=812