Silicone and Mastic Sealant
The most important part when selecting any sealant - whether it is the clear Silicone Sealant or any other type of sealant is to remember that it should be 'non toxic', especially if you are sealing round pipe work which will be in close contact with the pond water.
Unique underwater "silicone" sealer. This sealer looks like black silicone sealer but it sticks and sets underwater.
It is non-toxic and solvent free and can be used safely with fish actually in the tank / pond, fresh or saltwater. Sticks: - Butyl, rubber, PVC, wood, glass, rock, ceramics, concrete, vinyl, wood........