1 Scale inhibitor: it cannot ensure body health, and need special tools, so we do not advice.
2 Acid fluid wash: need to brush when heat pump working, damage to inner tank and pipes, not advice.
3 Softened water: reliable and good performance, but need special tools with timing salt, high maintenance cost, not available.
4 High frequency electricity: Complex installation, high cost, normal consumers will not accept.
5 GNM: Fixed frequency and strength, very low feasibility.
6 Electronic induction: Easy installation, no need maintenance, low power consumption, perfect solution. But also no promotion till now.
Maybe you will ask which one is better?
The above methods are not good, as there is dirty discharge port under water tank, you can use cold water to wash each three month or half an year, including heat exchanger and inner tank. However, if good quality water tank, the heat pump can be kept in good condition even be used for 8 years.