Backyard fish farming: How to turn your old swimming pool into a thriving koi or tilapia fish pond


Swimming pools get a bad rap in enviro-circles, and for good reason. They cost a great deal to construct – using a lot of CO2 intensive materials in the process – they waste huge amounts of water and energy for maintenance, use chemicals to keep them clear and ‘safe’, and they take up a lot of space that could be utilised for more productive purposes (like growing veggies!). Many people also just find them a lot of work to look after, which is especially annoying when their usage is often only seasonal at best.

But, what if you’re already lumbered with a pool and are trying to make the best of the situation? Maybe it came with your property, or hindsight has kicked in after you’ve shelled out thousands to install something you almost never use…. What then?

Some simply drain their pool of water and leave it at that. Although that is an option, it isn’t a particularly attractive one, and the pool interior will still require periodic sweeping and cleaning if you want to avoid raised eyebrows from your mother-in-law and other guests.

Is there anything you can do? Is there a way to use the pool while maintaining aesthetics and perhaps even bringing some other benefits with it?


If your swimming pool is no longer seeing much use, consider converting it into a fish pond. Swimming pools are the ideal size for koi or tilapia fish. You will need to eliminate areas where debris could collect, such as concrete steps. You must also use an acid wash to remove all traces of chlorine and other pool chemicals, as they kill the beneficial bacteria needed for a healthy, balanced, self-sustaining pond. Make a careful plan, rent a backhoe and get started. clean its interior with a natural substance such as vinegar and rinse it clean with water. If your pool is lined with plastic or metal instead of concrete, then the plastic or metal liner needs to be replaced with a wildlife-friendly liner. Opt for either concrete or a flexible synthetic liner installed by professionals.


Decide the shape, size and depth of your pond. You can keep the pool size and shape as they are if you choose. Straight sides, rectangular or ovoid shape with a sloping bottom are fine as is, although you may find it more aesthetically pleasing to modify the existing pool to give it a more natural look. Use sandbags to create a new inside shape. Backfill the corners of the pool with layers of gravel, sand and soil to create flower beds, or gravel alone to create biological filters. If you are going to make biological filters, run any electrical conduits, plumbing lines, air intakes and water outlets, then line the corner and fill with gravel. Do not use sand in your biological filters, as it can enter your pumps.


A filtration unit is essential for a pond, especially when the pond includes fish. It removes waste caused by fish and the breakdown of plants. Including a pump to circulate the pond's water is an option. A pump is beneficial because it introduces oxygen to the water, and both plants and fish need oxygen to survive. Also, if the pond water will overflow into areas of your yard during periods of rain, then figure out where that water will go, and plant moisture-tolerant plants in those areas.


A pond supports fish and other wildlife best when it includes plants; it may look best with plants, too. Because your pond will be lined, the easiest way to include plants in it is to use plants that grow well in containers and keep them in their containers in the pond. Container plants easily can be changed with other plants or replanted in larger containers as needed. Pond plant choices include common rush (Juncus effusus), which is hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9, “Sunny Pink” hardy water lily (Nymphaea “Sunny Pink”), hardy in USDA zones 4 through 10, and Chinese water chestnut (Eleocharis dulcis), hardy in USDA zones 9 through 11.


After your pond's filter is installed, your aquatic plants grow successfully and your pond's water has turned a greenish shade, fish can be introduced to the pond. Algae are responsible for the water's green color, and the presence of algae and healthy aquatic plants, along with an optional pump, ensures the pond's water can provide enough oxygen for fish. In order to attract birds and other wildlife, place boulders in the pond; the boulders provide places for the animals to rest and sun themselves. Other animals, such as butterflies, dragonflies, frogs and salamanders, will appear over time at the pond.

But remember…. Add fish one or two at a time when testing reveals no ammonia and no nitrates, waiting a week in between for the filters to adjust and the beneficial bacteria in your pond to respond to the increased ammonia load.