How inch of koi fish can you put in 40gallon tank?

A common guideline for stocking koi fish in a tank is to allocate about 10 gallons of water per inch of fish. Therefore, in a 40-gallon tank, you could theoretically keep around 4 inches of koi fish. However, it's important to consider that koi can grow quite large and produce significant waste, so it's often best to keep fewer fish to maintain water quality and ensure their well-being. Additionally, koi are typically better suited for larger outdoor ponds rather than small tanks.