A few points to note before we get down to actual numbers:
- With all types of fishkeeping, it is better to understock than overstock. Extra space is not a problem, so to ensure happy, healthy fish, err on the smaller size. Your fish may even reproduce, which could be a problem if you have stocked the pond to its maximum.
- Do not add all your fish in the first stocking of your pond. It is recommended you add the stock in small batches, in case there are problems. In other words, add 3-4 fish (scale up if you have a larger pond), wait and see if they are okay, then add more a few at a time.
- Different fish have different requirements. Koi are specialised fish and will need more space in a pond than goldfish.
When determining how many fish your pond can hold, both the surface area and volume of the pond are important. The surface is where oxygen transfer happens, so a deep pond with a small surface area will be able to hold less fish than a one with the same volume which has more balanced proportions. However, volume is also a factor: a pond that is too shallow can cramp the fish and not give them enough room to swim. Koi should not be kept in a pond that is less than 1 m deep.
This careguide suggests you allow 1 inch (2.5 cm) of fish per square foot of surface area. This other careguide from the same company suggests allowing 1 inch (2.5 cm) of fish for every 13 US gallons (50 litres) of water. Personally, I would compute the number of fish using my pond's surface area, then again using the volume, and use the smaller of the two numbers (or close to it).
Good filtration and water features could allow you to increase the fish capacity by up to double the (surface area) guideline above. Having more filtration will increase the number of fish you can keep because it will make the water cleaner as waste will be eliminated more promptly; water features will increase it because the water will get more oxygenated as the water bubbles and splashes around. However, these will only work up to a point - fish still need a certain amount of space no matter the cleanliness and oxygenation of the water.
As an alternative way of thinking about it, here is how the above careguide describes the different pond sizes. The measurements are US gallons.
- 1 to 5 gallon (3.8 to 19 liters) ponds – offer a carefree way to enjoy small water plants up close.
- 20 to 40 gallon (76 to 150 liters) ponds – get your feet wet. Provide a serene area for you to enjoy small to medium sized pond plants. You can also have a small pump and maybe even a goldfish. Even better – they’re easy to maintain.
- 50 to 500 gallon (190 to 1900 liters) ponds – a great place to start, not a big commitment. However, once you discover your enthusiasm for pond keeping, it won’t be long before you’re making plans to expand.
- 500 to 3000 gallon (1900 to 11400 liters) ponds – This size will allow you to keep fish and a large variety of pond plants. Consider location, filtration and the number of plants and fish that you want to keep when planning your pond.
- 3,000 gallon (11400 liters) ponds and over - many people find water gardening and fish keeping to be the hobby of their dreams and will invest the time and money to get the maximum enjoyment out of their water garden. With water gardening, the only limit is your imagination.
If you are interested in larger ponds or lakes, there is a guide here on how many fish to populate your lake with. It's probably outside the scope of pet keeping but might be interesting as further reading. (Bear in mind that bigger ponds can tolerate much more variance, so I wouldn't advise using those figures and "dividing down" for a smaller volume.)
