I often get asked what silicone is safe to use in a reef tank. I have personally used three types of silicone with no ill effects at all. The important thing to consider when selecting a silicone for use in a reef tank is to make sure that it is 100% silicone with no additives to eliminate mildew. Do NOT use a kitchen or bath silicone. I have never seen a kitchen or bath silicone that does not contain mildew protection.
What silicone brands are reef safe?
The three types of silicone I have found that meet these requirements are: All Glass Aquarium silicone, GE type I silicone and DAP 100% Silicone Sealant. Manufacturers tend to change their product names and ingredients, so always verify that the item is 100% silicone with no additives for mildew reduction.
What about long term use?
My 125 gallon reef tank has a 40 gallon sump that was assembled entirely with GE Type I silicone. My 46 gallon bowfront reef tank was resealed with black DAP 100% silicone sealant. I have had no ill effects from the use of these products and no problems at all with corals or invertebrates.