Here's a quote from Uncleof6,
"GE I Window and Door is the short cut stuff, but does not have the strength of the good stuff for one, and yes it is labeled not for aquarium use, however, too many people ignore the labels on things. The DAP "Aquarium Sealant", manufactured by Dow Corning, is the good stuff, but is inconveniently packaged rather small. CRL GE 1200, Momentive Performance Materials RTV 103 and RTV 108 (formerly GE silicones) Dow Corning 732 or 999A are the silicones of choice for serious tank building. The RTV stuff can be had from Aqueon/AGA in gun size tubes, as this is what they currently use."
For your C2C you can use GE I without any problems.