(Length x Width x Depth) x 6 = #Gallons
For example, our standard 8'x11' goldfish pond at 2' deep will come out to ~1056 gal.
Once you have an idea of how many gallons of water are in your pond, you can estimate the proper fish load. (Fish Load is the term we use to refer to the density of fish in a pond relative to the amount of water.) There are varying opinions on the optimum fish load for a koi pond, which is measured in inches of fish per gallon. These opinions vary from person to person and pond to pond. Some recommend no more than 3" of fish for every 100 gallons of water, while others say you can comfortably stock as much as 10" of fish per every 100 gallons of water. We have found that our ponds can sustain the upper limits of that range. The reason for the difference of opinion here involves many variables, the most important of which is the style of pond. Also factoring into the equation is the local climate, the amount of biological filtration from plants and bacteria, the amount of sunlight hitting the pond, the types of fish being stocked, and the stability of the pond ecosystem.
The type of pond makes such a big difference in how many fish it will support because that affects the other factors involved. The style we build here at Landvista Aquascapes is the Aquascape Ecosystem Pond. This style mimics nature, and has been thoroughly field-tested for peak performance. The key features of this style are an EPDM fish-safe liner, a skimmer, a bio-filter waterfall, rock and gravel construction, plenty of aeration, an array of aquatic plants, beneficial bacteria, and fish. This is a pond style that is designed to achieve a natural balance and require less tinkering than a traditional koi pond that uses exterior filtration, bottom drains, and sterilizers. Ponds that are lacking any of these elements are going to be less efficient, and will require a lighter fish load to stay balanced. Because the ecosystem pond has an innate harmony, it is able to handle a larger fish load than a tub pond or clay-bottom pond of similar volume.
For example, a pond that lacks a skimmer will accumulate debris and thus it will develop more algae. A pond without a gravel bottom will lack area for bacteria to colonize and result in less biological filtration, the same goes for a pond without the aquatic plants to consume nutrients from fish waste; both of those ponds will likely have issues with excess algae. A pond that lacks a waterfall or aerator will have lower oxygen levels, translating into less ability to support fish. It is still possible for one of those ponds to become a happy and healthy home for fish, the difference is that you'll probably need to do more frequent maintenance and the pond will find a lower equilibrium. These issues are more prevalent in smaller ponds than in large ponds, and that is simply due to the smaller volume being more sensitive to any changes.
Sometimes people have different goals for their ponds, and these people will want to take different approaches to stocking their pond. If your goal is to grow the biggest koi possible, then you need a pond that is large enough to give them room to swim around and develop their muscles. You'd also want to go with a lighter fish load (<6"/100gal) so they will have plenty of opportunity to grow. If you want your pond to support the greatest variety of fish that is possible, and you're less concerned with having them grow into jumbos, then you can use the upper limit of the recommended fish load, (up to 10"/100gal).
Another factor in determining fish load is the species of fish being stocked. These calculations are for koi/goldfish. In the short-term, both koi and goldfish have the same needs for space. In the long-term, koi will continue to grow after the goldfish plateau, so the same size koi will eventually need more space than an equal number of goldfish. If you want to keep more exotic/wild species such as catfish, sunnies or bass, they are happiest with extra space. For those fish, a good rule of thumb is to reduce the fish load down to half of what was recommended for goldfish or koi.
It can be quite tempting to fill your pond with fish way beyond the recommended amount. Some people even get away with double or triple the optimal fish load for a while. The problem with that is the ecosystem becomes strained and the fish become stunted. A pond under the strain of a heavy fish load has a delicate balance. A stressful event such as a power failure, a heat wave, visits from a predator, or trauma to the pond is much more likely to become overwhelming and result in tragedy for some of the fish. When fish are stressed their immune systems are weakened, and they become more prone to infections and parasites. The end result is usually a loss of fish and eventual restoration of balance.
Ultimately, it is your pond to enjoy and to manage however you please. Maybe you prefer birds over fish and find greater enjoyment in watching herons hunt for their lunch, in which case a heavy fish load would be a deliberately temporary condition. Maybe you want to experiment and discover for yourself just how far you can push the fish load in your pond before it becomes a problem. Maybe your pond is for raising tilapia or catfish, and you want to get the maximum yield, even if it means a lot of extra work doing maintenance. Whatever your preference may be, our goal is to educate you on why we make certain recommendations, and to provide the knowledge that will allow you to create your own vision of the perfect pond. Traditionally, we recommend a fish load that will enable long-term vitality in the pond, because in our opinion one of the greatest pleasures of being a pond owner is establishing a healthy micro-environment that continues to thrive and evolve year after year.