How Many Fish Should You Have in Your Pond?
The classic formula for fish stocking levels is one inch of fish per square foot of pond water. Ie, if you had a 10 foot square pond, you could have 100 inches of fish - about eight twelve inch fish (96 inches). Another classic formula is one inch of fish per ten gallons of pond water. By this formula, you could have the same eight twelve inch fishes in a 1000 gallon pond, or one twelve inch fish in a 120 gallon pond.
Well, those are easy-to-use formulas, but they don't address how much more waste fish create as they get larger. According to the old formulas, one twelve inch fish is the same as four three-inch fish. Not so. The twelve inch fish will weigh about 475 grams, wheras the four three-inch fish will weigh 29 grams all together. That's a big difference. Its even more significant when you take into account that each fish will generate about a third of its weight in waste every day. So your pond and your filter will have to accommodate 158 grams a day of fish waste from the twelve-incher, but less than ten grams of waste from the four three-inch fish.
So what does it mean? Well, it means that your small to medium sized pond can handle a bunch of small fish MUCH easier than it can handle even a few large fish. So with fish, smaller is better if you don't want to overload your pond. A pond three feet square - 1296 square inches - should not have more than 60 inches of goldfish; or about twenty goldfish. And that's with a seriously robust filter. Ten goldfish is much more likely to be a success.
Many koi experts use a rule of 1/2" of koi per square foot of surface area. This works better than the old formula, and also takes into account the fact that koi can get much larger than goldfish. Koi also tend to be messier than goldfish, and are quite a bit pickier about water quality. So under this rule, your 10x10 foot pond could take no more than four twelve inch koi. Remember, too, that koi and goldfish grow. It would be better to get four six-inch koi and let them grow than have to give one away in a year's time.
Speaking of which - if you have a new pond, do not load it up with fish all at once. You'll throw the biological filter off, have a severe ammonia spike, and be in a real risk of killing all your new fish. Instead, start with one (two, maximum) fish, let the pond and the fish adjust for at least two weeks (a month or more is much better), then add other fish. Your pond is, after all, a delicate ecological system - for it to function properly you've got to give it time to adjust to the bioload of new fish.
Before we close, one word about filters. They should be powerful enough to cycle all the water in your pond every hour, and you'll need a biological filter that can handle the fish load. This is typically measured in SSA, or specific surface area. As an example, Bio beads, a common biological filter media, have a 58 SSA per square foot. A three square foot filter of bio beads can handle about 31 grams of fish waste. That's approximately the waste created by one seven inch fish, or twelve three-inch fish.