A simple question with a not-so-simple answer. This is because there are many factors to determine a good population for your koi*pond. Now, some rules of thumb are out there based on gallons of the water of the pond. The answer you will find for grown, developed koi will range from basic rule 250 gallons per koi, up to 1,000 gallon + per koi. Please note: if you get small koi, they WILL grow to become a BIG KOI before you know it ! It is not very smart to think that the gallon amount of water alone is all there is to creating a heathy environment for your koi to thrive in. There are a multitude of variables that will impact the needs in creating a heathy environment for your fish. A biological ecosystem – yes that is what you are creating- has a multitude of factors seen and even unseen. Good Filtration and circulation are essential to this equation. Furthermore, there are many other considerations that come into play, such as temperature, climate, depth of the pond, and aeration.
THE BEST ADVICE IS TO STOCK THE POND CONSERVATIVELY AND TO MAKE A WELL ADVISED DETERMINATION BASED ON YOUR INDIVIDUAL POND. Sadly, most novice koi keepers think too BIG in terms of their ponds ability to support and consequently learn the hard way that they have overstocked; ammonia spikes, fighting disease, overcrowding …sadly losing fish by death till they finally determine just how many koi their pond can support. Thus, our advice is go slowly, collect over time and avoid overwhelming your pond.
*Note; Other breeds of fish such as goldfish or minnows are much smaller and thus have much smaller impact per fish on the ecosystem and thus you can stock a vastly larger number of fish. We recommend smaller fish breeds for any shallow pond (under 24 ft. deep) or under 2,500 gallons.