Understanding biological load and avoiding overcrowding is the most important skill for a successful water gardener.
When standing in a pet store looking at small, 3-inch Koi or Goldfish, it is incredibly easy to buy too many. The most common mistake new pond builders make is failing to account for how large these fish will grow and how much biological waste they will produce as adults.
Fish constantly excrete ammonia into the water through their gills and waste. Beneficial bacteria in your biological filter consume this ammonia and convert it into less harmful nitrates. If you have too many fish (a high "Biological Load") or a filter that is too small, the ammonia levels will spike rapidly, burning the fish's gills and eventually killing them.
Different species of pond fish have vastly different requirements for swimming space and water volume dilution to handle their waste output. Always stock your pond based on the expected adult size of the fish, not their current juvenile size.
| Fish Species | Expected Adult Size | Recommended Volume (Per Adult Fish) |
|---|---|---|
| Koi (Japanese or Domestic) | 24 to 36 inches (60-90 cm) | ~950 Liters (250 US Gallons) per fish |
| Sturgeon | 36 to 60+ inches (90-150 cm) | ~2,500 Liters (660 US Gallons) per fish |
| Golden Orfe | 18 to 24 inches (45-60 cm) | ~190 Liters (50 US Gallons) per fish |
| Comet / Shubunkin Goldfish | 10 to 14 inches (25-35 cm) | ~115 Liters (30 US Gallons) per fish |
Koi are often referred to as "water pigs." They eat aggressively, root around in plant pots, and produce massive amounts of waste. A pond intended for Koi should ideally be at least 4,000 Liters (1,000 Gallons) in total volume and have a minimum depth of 1.2 meters (4 feet) to protect them from predators like herons and provide stable temperature insulation during winter freezes.
Because Koi grow so large and fast, the ~950 Liters per fish rule is a strict minimum. Professional Koi keepers often advocate for closer to 1,500 Liters per adult Koi to ensure pristine water quality and maximum growth potential.
If your pond is smaller than 4,000 Liters, you should strongly consider stocking Goldfish (Comets, Shubunkins, or Sarasa) instead of Koi. Goldfish are incredibly hardy, colorful, and produce significantly less waste. A healthy Goldfish can still reach over a foot in length, providing an excellent visual display without overwhelming your filtration system.
Sturgeon are ancient, fascinating bottom-dwellers, but they are not for beginners. They require massive amounts of highly oxygenated water, very large swimming areas without tight corners (as they cannot swim backwards), and specialized sinking food. Because their gills process oxygen differently, a pond pump failure on a hot summer day can kill a Sturgeon in a matter of hours.
Are you worried that you might be overstocking your new pond? Use our built-in Fish Viability Simulator!
First, open the Pond Sizing Calculator and draw your pond. Once the volume is calculated, click the "Test Viability" button on the Fish Capacity card. You can use interactive sliders to mix and match different species, and the engine will instantly tell you if your water volume can support the biological load.