Most kitchen and bath caulk has arsenic in it and will kill your tank inhabitants. Buy aquarium caulk. For small amounts most fish stores stock the small tubes. For larger projects order it online. Most large lumberyards sell dozens of types of caulk and yet few if any are aquarium safe. You have to get caulk that says aquarium use or food safe. Even some that say 100% silicone aren't safe to use. The tube you're holding up says mold free and that means it has an additive to kill algae. Most likely arsenic.
It's a glass sump . Dang I didn't see all these replys and I applied the Silicone but it was on the out side on the bottom part of the tank .will the Silicone leach in to my tank and harm it
If water was leaking out where you applied the silicone then it's coming in contact with the tank water. It may not be enough to affect your tank but I wouldn't risk it. It's important to know how safe anything is you intend to use in a marine tank.