Breeding koi and goldfish in ponds is so easy, it would be difficult to keep them from reproducing. Here's what you need to know about healthy Pond Fish Breeding and Reproduction.
A Place to Call Their Own
The first thing you'll need to do to encourage spawning is provide attractive spawning media, or places where the fish will want to lay their eggs. They like to spawn on things dangling in the water such as pond grasses, water hyacinths, nylon mops and splayed rope segments.
Gold fish will begin spawn when they are two years old. Koi need to be three or four years old to spawn. Keep in mind that water and food quality will greatly affect spawning cycles. Females need plenty of protein to produce healthy eggs. To be sure you have both boys and girls in the pond, look for differences in body shapes. The males tend to be more of a torpedo shape while the females are rounder, especially around the abdominal area.
The eggs will appear as jellied blobs in the pond. There will be many eggs, but the fish will eat a lot of them unless you put them in a separate pond. Carefully monitor the water quality to keep them incubating well. A couple days after the fry hatch, feed them a commercially prepared fry food in small quantities several times a day. Do not overfeed them! Within a few weeks the fry will begin eating crushed pelleted food. At that time you will want to cull out the weakest specimens for the betterment of the school. Any deformed, colorless or slow-growing fish can be removed, placed in a pan of ice water and frozen.
Pond Species 
Pond Fish Health - Medical 
