The answer is a bit more nuanced than that. GE2 is a neutral cure silicone that is formulated to work better on a wider range of materials, including plastic. If allowed to cure properly (at least a week) it is reef safe. GE1 is an acetoxy cure silicone (the vinegar smell) that is formulated to work better on glass and ceramic. It still must be allowed to cure properly to be reef safe. Maybe not a full week, but at least 72 hours depending on bead size. Any silicone, if uncured, will kill your tank. Since I work primarily with glass, an acetoxy cure silicone is what I use (ASI silicone in black with a 7-10 day cure).
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. 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.)
Co-polymers, are another category (not 100% silicone) and are out of the scope of this thread.
Within the two general categories, there are several sub-categories: sealant, adhesive, high temperature, etc. Obviously we are only concerned with sealants and adhesives. the choice of which to use, really takes little thought. Sealants seal, adhsives hold stuff together.
GE Silicone I (Momentive) is, easy to work with, but is a bargain basement low end sealant. RTV100 series (Momentive) or SCS1200 (Momentive) are high end adhesives, not so easy to work with. In between there is a plethora of formulations, that will drive the most studious nuts ( I am a prime example.)
A short list has been publised at least a 100 times here on RC, not that they are the only choices, but everytime some off the wall silicone is asked about, or mentioned, it just adds to the confusion (and rumors.) In similar fashion "what is safe and unsafe" has been published at least the same number of times, over the past several years, here on RC.
