Hobby Silicone Black 200ml

can also be used to fix hardscape into place 

The universal high-performance sealing and bonding adhesive suitable for use both above the water and beneath the water.
The underwater adhesive cartridge is particularly easy to use, setting new standards in adhesive and bonding processes. Take the cartridge in your hand, press the lever with your thumb and apply the underwater adhesive in the required thickness.
It couldn't be simpler!
The underwater adhesive is free of solvents, isocyanate and silicone and is therefore harmless to all animals and plants. The product is compatible with marine water and can also be used to stick and repair pond liners. The underwater adhesive is ideal for sticking down decorations and ornaments and for repairing leaks in aquaria or terraria. A leaking aquarium is a nightmare for anyone who keeps an aquarium, especially after the shops have shut or at weekends or on public holidays. The adhesive is therefore an essential item in the "emergency kit" in any household with an aquarium.