I have hard water. Should I dose APT SKY?Updated a month ago

Generally you should not need to add APT SKY.

Hard water simply means your water has high mineral content.

This can mean either: 

(a) a high level of Carbonates (high KH reading) or

(b) a high level of Calcium / Magnesium (high GH reading) or

(c) both of the above.

It is common for hard water to have both high GH and KH because sources of hard water usually come from areas with limestone [CaCO3] which contributes to both GH [Ca] and KH [CO3]. However, hard water can be high in GH and low in KH and vice versa.

Basically, if you have GH below 5 dGH, you can use APT SKY to raise GH to the recommended ~5 dGH zone.

However APT SKY does not alter KH, so simply raising GH to the ~5 dGH zone will not allow you to keep soft water plants if you have high KH.

Read more about soft water and hard water here. 

Tanks that make use of Seiryu rock, a type of weathered limestone, will generally have elevated hardness levels. Soft water plants are not compatible with such rocks long term. In the short run, acidic aquasoils can absorb carbonates and keep the KH low. As the aquasoil's buffering capacity exhausts, the KH in such tanks will inevitably rise.