Once Silicone I is cured it no longer gives off acetic acid, and once Silicone II is cured it no longer gives off ammonia. In either case, it is very important to let silicone cure completely, but since ammonia is a much stronger fish toxin than acetic acid, this becomes even more of an issue with Silicone II. Once cured, neither Silicone I nor Silicone II pose a health risk to your fish. GE Silicone I for Door and Window has been used by so many aquarists with excellent success for so many years that there can be no reasonable doubt that it is aquarium safe.
Personally, I have installed rock backgrounds in a 29G tank and a 240G tank using GE Silicone II for Door and Window. These backgrounds consisted of real rocks that were attached to the back glass of the tank using silicone. Due to the irregular shape of the rocks, copious amounts of silicone were needed to complete the project. If memory serves, I used about 5 cartridges of silicone for the 29G, and more than a case of silicone for the 240G. Since the silicone is applied very thickly in places, many days should be allowed for complete curing, but in both cases no ill effects on fish, plants, or invertebrates were observed in many years of operating the tanks - at the time of writing this article (Jan 2013) almost eight years for the 240G.
Photo from manufacturer's website. Added by Cichlid-Forum for illustration purposes.
Door and Window vs. Kitchen and Bath - potentially the big difference!
Silicone for Kitchen and Bath tends to contain biocides, which are toxins that leach into the water and prevent the growth of mold, mildew, and bacteria. Silicone for Kitchen and Bath is not pure silicone, but a silicone product with additives that act as biocides. In a 2011 press release, GE lists only its Kitchen and Bath Silicone as allergen fighting, biocide containing, with five to ten year mold-free protection. These additives have been blamed by many aquarists who have experienced fish loss after completing aquarium projects with silicone. Both GE Silicone I and Silicone II are available with biocides (kitchen and bath) and without biocides (door and window). Personally I have never used silicone with biocides for aquarium purposes, and I would not recommend it.
Until recently it was fairly easy for the aquarist to select an aquarium safe silicone by avoiding any product that was labeled as mold and and mildew inhibiting. Unfortunately these seem such irresistible properties for the homeowner that GE has begun to market all of its silicone products with some sort of anti mold message, not necessarily because they leach toxins, but because they form a smooth and durable surface that prevents the formation of mold. For example GE Silicone II for Door and Window now carries a 'mold-free product protection'. Although I have not tried the latest iteration of that product in a fish tank, I believe it is still aquarium safe.
GE has also caused some confusion amongst animal keepers with it's BioSeal label. The jury is still out on whether this refers to biocides or simply a superior surface.
100% Silicone - does this label mean I have pure silicone?
Silicone with biocides contains more than 99.99% silicone and only a tiny little amount of additives - enough to kill all of your fish, but not enough to stop the product from being marketed as 100% silicone!
The 100% silicone label is used only to distinguish silicone from siliconized-acrylic caulk. Silicone is an adhesive that can also be used as caulk. In fact, it has a number of properties which make it very desirable as a caulk - permanently waterproof, flexible, shrink-proof, and crack proof. Acrylic caulk and siliconized-acrylic caulk are far cheaper than silicone, and serve the homeowner for various sealing projects. Those caulks lack the adhesive strength of silicone to hold glass tanks together. They are generally not useful for aquarium purposes where we require an adhesive, not a caulk.