Delaware Custom Ponds LLC

Alkalinity optimum range: 4-10 dKH, or 72-180 ppm. Water in the low end of this range is considered soft, while water in the high end is considered hard. Koi do just fine in either, provided your levels remain stable over time

Optimal water temperature for maximum koi growth and activity level: 75 deg F. This is my personal observation; other people may come out slightly higher or lower but likely still somewhere in the 70’s deg F

Minimum water temperature koi spawning: 60 deg F. They can go at it anytime from spring through fall. Good luck trying to stop them!

Minimum undisturbed water temperature below ice in winter: 39 deg F. Water has a somewhat uncommon property among liquids in that it freezes from the top down. Water molecules reach maximum density at 39 deg F, which means that regardless of air temperature, below the ice the lowest level of undisturbed liquid water will remain stable at that temperature (unless you supercool it, which we’re not going to discuss here)

Aeromona alley: 45-60 deg F. Danger zone in late winter and early spring in temperate climates when koi are most vulnerable to aeromonas and other pathogens. Koi immune system performance increases with water temperature, and at these cooler temps some pathogens like the dreaded aeromona may be more active than your koi

Nitrates: aim for under 25 ppm. You can get away with going higher to 50-75 ppm, but the lower the better. If you get to zero you win a prize

Phosphates: under .05 ppm. This represents levels found naturally in healthy water systems, though it may be unattainable for some hobbyists. Bioavailable water column phosphates are the primary driver of pesky nuisance algae, which is why so many algae control products specifically target this

Ammonia: zero ppm

Nitrite: zero ppm

Free chlorine: zero ppm

Dissolved oxygen: above 6 ppm. This is a minimum, higher is better. Also note that cold water holds more oxygen and other dissolved gases at saturation than warm water, so keep an eye on this particularly during the hot summer months

Methane & hydrogen sulfide: if your pond smells foul you have a problem

All of these parameters listed above are both critical and fairly easy to observe or test for yourself. If you suspect problems with heavy metals, trace elements, or anything else, you will likely need to send out for a full ICP test.