Several years ago, while trying to get info on RTV Silicone from several manufacturers, I called GE's 1-800 number, and was told they wouldn't stand behind their product for use on aquariums larger than 5 gallons. While I think this has more to do with liability than actual quality of product......it wasn't exactly confidence inspiring. So, next up was Dow-Corning, and the responses I got back from them indicated their reccomendation was Dow-Corning 732. This product is generally stocked by Henry @ Nature's Corner, and a number of Industrial jobbers as well.
HTH
James