I've used General Electric's Silicone I caulk adhesive. It is 100% silicone
Be careful not to confuse SIlicone I with Silicone II. Silicone II is designed for waterproofing kitchens and bathrooms. It has additives that prevent the growth of mold. I don't use this because I don't know if the additives effect fishies and never wanted to risk it given the stunning success of Silicone I.