I'm currently using a drop checker to measure my co2.
As per instruction for the drop checker, I mixed the regent with the tank water till it filled half a ball.
Then have read more on this topic online that it will give inaccurate representation if I'm using tank water as a mixer due to soil contents.

So should I be using just normal tap water or what?

If I want to use kh/pH values to further supplement verify the drop checker readings... How exactly do I do it?