3 min read
a backyard koi pond

Fish is part four of six in the Ornamental Garden Pools series.

One common mistake made by novice ornamental pool owners is to stock too many fish. Many people do not consider the number of fish the pool can safely support and that fish grow. A pool is suitable for fish as long as it can supply adequate oxygen and decompose the wastes. Most of the fish commonly placed in ornamental pools grow rapidly and may keep on growing unless they are kept on a limited diet.

Pool carrying capacity, or number of fish the pool can support, varies, depending on a variety of factors. These factors include size of pool, temperature, amount of sunlight the pool receives (which influences oxygen levels), whether aeration is provided, and how well the filtration system removes wastes. The stocking rate, or number of fish to put in the pool, should not exceed the pool’s estimated carrying capacity. The following examples give stocking rates recommended by fish hobbyist magazines.

First, determine the pool’s surface area in square feet. Stock an unaerated pool with one 12-inch fish (not including the tail) per 4.5 square feet of surface area. Stock an aerated pool with one fish per 2 to 3 square feet. Conservative hobbyists suggest stocking only 1 to 6 inches of fish per 5 square feet of surface or 8 to 12 inches of fish per 16.5 square feet of surface area.

For example, if a pool measures 9 x 15 feet, the surface area is 135 square feet. Dividing 135 square feet by 4.5 square feet = 30, the units of 12-inch fish bodies per unit, or 360 inches total. The average adult koi carp is 18 inches, so this 9- x 15-foot pool can support 20 adult koi carp. Top exhibitors in Japan stock only 10 to 15 koi even in large pools, while the majority of koi owners tend to slightly over stock.

Large koi carp swimming in water

Kinds of Fish

Fish commonly stocked into ornamental pools belong to either the goldfish or the koi family. There are numerous varieties of these fish.

Early references to the common goldfish are found in Chinese poetry as early as 1,000 A.D. The Chinese and Japanese nobility led the way in developing many of the varieties we see today, most of which had been developed by the sixteenth century.

Common and fancy goldfish are the same species (Carassius auratus). The common goldfish, shubunkin (or calico), comet, and fantail goldfish have body structures similar to the wild form. What makes these varieties distinct are differences in coloration or fin shape. Members of the goldfish family, which vary from the wild form in both body structures and fin shape, include the nymph, fringetail, and veiltail goldfish. Additionally, there are strange varieties that have markedly altered bodies bordering on the grotesque, such as the eggfish, tiger-head, lionhead, circled gill, and bubble-eyed goldfish.

Koi carp are descendants of the European common carp (Cyprinus carpio). Koi is a Japanese word meaning “love,” and koi-giving in Japan approximates the meaning of flower giving in the West. Koi have been bred in Japan since at least 300 A.D. They are a very popular ornamental fish because of their wide variety of colors and color patterns. Each color and pattern combination on the koi is given a distinctive Japanese name. The most prized koi have the brightest, most intense colors, the sharpest color definitions, and the most distinctive placement of markings. Koi with exceptional coloration and patterning can be valued at thousands of dollars per fish.

Pool owners should be aware that koi can grow quite large and sometimes live for 60 or 70 years. These fish can truly be lifelong pets and then be passed on as family heirlooms.

Acclimating the Fish at Stocking

First, inspect the fish carefully before purchase to be sure that they appear healthy (see disease and stress sections). Fish must be stocked correctly if they are to remain healthy. When stocking, it is necessary to acclimate the fish to the temperature and pH of the pool water. To do this, float the transport bag containing fish in the pool for 10 to 15 minutes. Keep the bag out of direct sunlight, because a plastic bag acts like a magnifying glass and will rapidly heat the water inside. Next, open the bag and slowly splash water from the pool into the bag. Check the water temperature in the bag and pool with a thermometer or your fingers. Once the water temperatures are the same or within 2 or 3 degrees, release the fish into the pool. It is best to dip the fish out of the bag to eliminate adding the transport water to the pool. The fish should swim away and behave normally.

Ornamental Garden Pools Series

Did you find this helpful?