Completely drain the pool of its water. The water used in pools contains chemicals designed to kill all living bacteria. These chemicals will also kill any fish you put into the pool. If your pool does not have a drain, place a siphon pump's hose into the water and pump the water out of the pool.
Remove and discard all of the filters used to filter your pool.
Put on rubber gloves, long-sleeve shirt, long pants and goggles. Mix muriatic acid with water in a 5-gallon bucket according to label directions. Scrub all of the mechanical equipment and the pool lining with a nylon-bristle brush and the acid. After scrubbing everything, hose off with clean water and scrub it a second time with muriatic acid. This acid removes all of the chemicals used in the pool.
Replace the existing water pump for your pool with a high-volume low-pressure pump. Most pools use high-volume high-pressure pumps that could hurt fish. You can connect it to the existing ductwork for your pool to save money and make pump routing simple. Since the pump fits in-line with your existing ductwork, you can complete the task on your own. However, if you are uncomfortable with the task, consult a pool-installation company and have it complete the work.
Install a submersible pump in the center of the pool to push water upward, if you live in an area where the water could freeze. A pump that pushes water up, like in a fountain, will keep the surface from freezing because it keeps the water moving constantly. Do not rely solely on the drain in the bottom of the pool to keep water from freezing on the surface.
Replace the filter media in your pool filters with those suitable for ponds inhabited by fish. Standard pool filters will fill up and clog quickly. The use of filters designed specifically for ponds is best.
Add a rocky substrate to the bottom of the pool for bacteria growth and to give the pool a natural look. Don't worry about the sides of the pool. Algae will grow quickly and cover the sides of the pool. Add landscaping around the pool to give it a natural edge.