There are a lot of different types of epoxy and silicone that are reef safe, and plenty that aren’t. For example, Silicone 1 is reef safe, but Silicone 2 isn’t. The reason Silicone 2 isn’t reef safe is due to its mildew inhibiting chemicals.This silicone was designed for kitchen and batch applications, but not saltwater aquariums. If you are ever in doubt, ask around on aquarium forums or look for silicone specifically designed for aquariums (will indicate this on the packaging).
Tip of the Day 11/8/100