Water Hardness
consists of two elements, permanent or general hardness and temporary or carbonate/bicarbonate hardness. Koi do better in hard water because of the relation of salt within their bodies to the dissolved salts in the pond. In soft water, the difference in salt concentrations means the koi have to work harder, through the process of osmoregulation, to prevent the salts within their bodies from diffusing out through their gill membranes. Harder water allows the koi to ease up on osmoregulation and therefore reduce stress. As mentioned above, bicarbonate ions buffer the water, reducing pH shifts, another cause of stress in koi. Koi do well in carbonate hardness of 150-300 mg/liter or 9-18 degrees dll. In most koi ponds the water is too soft due to the fact that there is no natural mud bottom that leaches minerals into the water. Marine salt and sodium bicarbonate increases hardness and will also cause pH to go up. A permanent salt solution of 0.1% is beneficial to koi, and works out to eight pounds per 1,000 gallons. Check your pH if you add salt, and do not use table salt. Salt will not evaporate out and needs to be replaced only if water is drained from the pond

The water was treated to keep it soft, making it very good water in which to raise koi" by Ken Sasaki.
The purification system used are "Izueki Net" and "Magic Stone". It treated iron and hardness to 0.02ppm and 1ppm respectively.

Water Hardness
Hard water is due to an abundant number of salts such as calcium and magnesium. Koi can cope with a wide range of hardness. There are two major benefits to having hard water in your Koi pond. First, very hard water can bind some toxic metals such as lead. Secondly, hard water reduces the workload of the koi for osmoregulatory functions. A reading of 0 to 75 PPM is considered soft, from 75 to 150 PPM is moderately hard, from 150 to 300 PPM is hard and above 300 PPM is very hard. If it is necessary to increase your hardness you can add crushed oyster shells, coral or any substance that will increase the amount of calcium.