When selecting fish to add to your pond, it's easy to get carried away and overstock your pond. Fish need room to grow and swim. Adding too many fish at the same time can end up causing a big problem later on.
Rule of Thumb : To account for all kinds of fish, keep your fish load under one-inch of fish length (excluding the tail fins) for every one square foot of water surface--about one goldfish per 3 to 4 feet of water surface area. Because Koi grow larger, place one Koi to every 10 square feet of surface area. Typically Koi should be in ponds that are at least 1000 gallons. Once the pond is built, pond owners can add a few fish at a time over a 30-day period, allowing time for the pond and filter to be biologically ready to support the additional aquatic life. A mature pond can support two to three inches of fish per square foot. If your fish population exceeds these guidelines, you may need to increase the size of your filter.