The number of fish in your pond greatly affects pond health. Too many fish leads to decreased oxygen levels and the extra fish waste can result in ammonia and nitrite build-up. With an average-sized filter, your preferred stocking level will be based on the surface area of the pond.
There are different guidelines for stocking density, depending on the desired maintenance level of your backyard pond, and how many fish it was designed to support (filtration, predator protection, etc.)
If you want a low-maintenance pond, you should calculate a ½” of fish per ten gallons of water. This will allow you to have a simpler filtration system. The highest level of stocking that we recommend is calculated as 1” of fish per ten gallons of water, but keep in mind they will grow larger over the years. You can have a few more goldfish than koi, should you wish. You can also mix the two, substituting two goldfish for every koi. There are many lovely choices of both!
Remember, too many fish in the pond creates an imbalance in water, so you’ll want to make sure you’re smart about the number and size of fish that you place in the water gardens you create and install. So no matter how tempting it might be to add just a few more fish, be careful not to overstock.