Scrub your old bathtub with a nylon brush and soapy water. Remove any greasy or oily spots. Grease and oil can leech into water and kill vegetation or aquatic animals. Rinse well with clear water from a garden hose. Let it dry completely.
Seal the overflow drain with silicone sealant. Leave the main drain unsealed for now. You will address this later. Let the sealant dry overnight.
Place the empty tub where you want it. Place it above ground on your deck or in your garden if it is a claw-foot tub or one that has an attractive exterior. If you prefer, dig a hole slightly longer and wider than the tub. The depth of the hole should be the same as the tub. Place the tub in the hole with the rim at ground level. Use a carpenter's level to straighten the tub in the hole. Fill in around the tub with dirt and pack well to secure it in place.
Place a flat rubber stopper over the main drain in the bottom of the tub. The weight of the water will keep the stopper in place. In above-ground ponds, you will be able to remove it later to help with cleaning.
Fill with tub with water. If you are going to add fish or amphibians to your pond, treat the water with aquarium water treatment. Follow label directions for amount. Let the water stand for the recommended time.
Place a brick in the bottom of your pond. Set a pond pump on the brick that is of sufficient capacity for your bathtub. Plug in the pump and adjust it so that it circulates and aerates the water.
Landscape around your bathtub pond if it is sunk in the ground with decorative rocks and ground cover plants. If it is above ground, arrange potted plants around your pond. Add aquatic plants, fish or other aquatic animals to your pond.