GE1 only.. if it says mildew inhibitors, that is what you want to stay away from.
I guess I’m gonna need to build my new sump (on video) with GE II with mold and mildew inhibitors. I already built one with it. Here’s what we know. This is from one of the best tank builders I’ve known. See below:
For the record: All silicones are reef safe-once they are fully cured. The enemy is uncured silicone, not some mythical ingredient for controlling mildew.
That said, GE Silicone I is cheap junk that should only to be used for sump baffles or the like. It is the aboslute barrel bottom of all materials (and many silicones are right there with it.) For our projects we really should move up to a higher quality product. Dow Corning makes several that blow GE Silicone away, but are not as hefty as the RTV100 series or SCS1200, however for small projects they are fine. DC 735, DC 999A to name two.
The MSDS for Silicone II does not indicate that there are any additions other than siloxanes, of which there are literally hundreds, (polymers) common to all silicones, without getting to complicated about it, The MSDS also does not indicate any proprietary additives either. Where such is present, the law requires that they be included in the MSDS. This raises serious questions as to the validity of the advertising.
What differentiates Silicone II from Silicone I is the latter is an acetoxy cure silicone, and the former is a neutral cure silicone.
Silicone II does not take longer to cure; published cure times are about the same. However the cure times pertain to a specific amount of silicone, usually given in a bead size, generally 1/8" or 1/4". Hardly have I ever seen an aquarium application use that little. As the amount increases the cure time increases, a week, two weeks...RTV100 series' 7 day full cure time is for a 1/4" bead!
I would say that neutral cure silicone is a bit more toxic in its "uncured" state than acetoxy cure silicone, but that is as far as it goes. It is better to malign a product for what it does not do well, rather than internet rumor.
Acetoxy cure silicone excels at bonding to glass and ceramics; acetic acid is corrosive, so it is not suited for metals, and it does not bond well to other materials (acrylic included.) Neutral cure excels at bonding metals (non-corrosive) and other materials (acrylic not included.)