I just spent the past hour or so of my precious and too-short life Googling to find something definitive on this topic. Some of what I found was contradictory, most was confusing and inconclusive.
Here is a synopsis:
There are a bunch of things that are added to silicone sealants and adhesives including fillers, plasticizers, adhesion promoters, cross-linkers, and catalysts. In some specialized circumstances, iron oxide or even platinum is added (heat resistance, fire resistance).
The DAP Aquarium Sealant I bought from Home Depot doesn't have any ingredients listed on the tube, but I found the
MSDS (Material Safety Data Sheet for those unfamiliar with the term). The listed ingredients are:
<=87.0% Dimethyl siloxane, hydroxy-terminated
<=10.0% Silica, terminated
1.0-5.0% Methyltriacetoxysilane
1.0-5.0% Ethyltriacetoxysilane
I had less success with the GE Silicone I and Silicone II. It seems that GE has a fairly definite idea of intended markets, and insists that these products are for household (not industrial) use, and therefore don't require an MSDS. Their industrial silicones site doesn't list either of these products, so it is hard to draw any conclusions. One thing I did notice is that the
Silicone II for Kitchen & Bath contains fungicides and is definitely out of the question! See this for more information.
C.R. Laurence also makes aquarium sealant. Theirs is available in 52 gallon drums for those really big projects. The
MSDS for that product lists the following:
60.0-80.0% Dimethyl siloxane, hydroxy-terminated
10.0 - 30.0% Silanol/STPD Siloxane W/ME SILSQXNS (whatever that means)
7.0 - 30.0% Silica, amorphous, fumed
1.0 - 5.0% Methyltriacetoxysilane
1.0 - 5.0% Octamethylcyclotetrasiloxane
Big Al's Online sells All-Glass Silicone Sealant in 1, 3, and 10.3 ounce sizes in clear, and 3 and 10.3 ounce in black, ranging in price from $2.99 to $7.99. I checked out the
All-Glass Aquarium website, but found no list of ingredients.
In conclusion, I would recommend using a product that is specifically recommended for aquarium use. I know this means I am contradicting what I said earlier, but then again, I did my (and your) homework this time. The difference in cost, especially if you only need a little bit, is negligible in the long run and there is additional peace of mind in using a product when the manufacturer is willing to accept the potential liability and is stating that the product is safe for this purpose.