The maximum number of fish you can have in a pond depends on the volume of the water, the quality of the water, the size of the fish, the species of the fish, the climate of the area and the food available.
If the pond has no pump, filter or other equipment, in a warm climate, you shouldn’t keep more than two 15 centimetre fish per cubic metre (two 6 inch fish per cubic yard). You can keep fish at much higher densities than this, but the chances of them remaining healthy are not good.
The better the water quality, the more fish you can keep. With the right technology, fish farms keep up to 200 kilograms of fish in a cubic metre of water (about six pounds of fish per cubic foot). However, as a rough guide, in a garden pond half a metre (one and a half feet) deep, constantly in the shade, with well-buffered water being pumped continuously through an ultra-violet clarifier (to prevent algae blooms) and a biological filter, you could keep up to nine goldfish of 15 centimetres (six inches) length per square metre of pond (i.e.135 centimetres of fish per square metre, or about 45 inches of fish per square yard). To be safe, keep less than half this number so that you have plenty of leeway in the event of the pump or UV failing. In a warm climate, if the pond is in the sun much of the day, reduce the number by half again – i.e. about 35 centimetres per square metre (14 inches per square yard).
Obviously, the larger the fish, the more it pollutes the water so the less the number you can have in the pond. Fish growth tends to be restricted by the size of the water body they live in. For example, goldfish can grow to more than 30 centimetres (12 inches) long, but in most average-sized ponds, they usually won’t grow to more than 20 centimetres (8 inches).
Some species need more space than others and some species pollute the water more than others. For example, koi are twice as polluting as goldfish, probably because they stir up the pond bottom sediments more. Also, some species of fish are more tolerant of poor water quality than others. For example, koi can live in water that isn’t pure enough for trout.
Climate affects fish activity because they are cold-blooded. Their metabolism doubles with every 10 degrees centigrade rise in temperature which means they eat twice as much and pollute the water twice as much. As a general rule, you can keep double the number of fish in water at 13 degrees centigrade as you can in water at 26 degrees centigrade.
Ideally, you should keep only as many fish in a pond as the food chain in the pond can support. The amount of natural food available will vary from pond to pond. Up to 30 centimetres per square metre (12 inches per square yard) of fish survive well in some ponds without being fed. So, keeping the fish numbers down means you don’t have to spend the money or time to feed them. More importantly, by not feeding the fish you aren’t adding nutrients to the pond which feed algae.