Brick ponds are an attractive formal feature in your garden, but because concrete and brick are semi-porous materials, you can have problems with water seeping into the walls if they aren't properly sealed. While chemical sealers are available for coating the walls of the pond, that still leaves the base of the pond open. To correct this problem, the only real solution is to install a plastic pond liner into the brick pond, allowing the brick to be a decorative and structural surface, while the actual work of holding the water in place is done by a sheet of thick plastic.
Fill the base of the brick pond with 10 cm (4 inches) of sand to provide a soft, stable base for the pond liner.
Unroll the pond liner on the ground near your pond.
Apply several heavy beads of construction adhesive to the inside of the brick pond, making sure to apply a full bead all the way around the top inside edge of the brick. The construction adhesive must not only be waterproof, but also able to bond PVC to brick.
Place the pond liner on top of the pond, then create a slight depression in the middle by pressing it down 15 to 20 cm (6 to 8 inches) with your hand.
Fill the depression with water while feeding the PVC liner over the edge of the brick to ensure it does not catch on anything. As the depression fills with water, the liner will be drawn into the brick pond. Once the pond has completely filled with water, the water pressure in the pond will press the PVC liner against the glue and the brick, where it will set.
Cut the excess pond liner that hangs over the edge of the brick pond with a pair of scissors.
For a clean appearance, cut the pond liner 2.5 cm (1 inch) above the level of the brick, fold it over the edge and install another course of brick. This will hide the top edge of the pond liner and also hold down the folded inside edges of the liner.