Using the chart isn't going to work in your case because you have something else (your substrate) altering the pH of your tank.
I am wondering what kind of soil it is that would cause your pH to be that low... and with a KH reading as low as yours, altering the pH will be no easy task as you don't have a lot of buffering capacity. You have to be mindful of causing any large pH swings, which would be more harmful than having the low pH to begin with.
Since your test kit bottoms out at 6.0, I am wondering what your actual measurement is.
I am wondering what kind of soil it is that would cause your pH to be that low... and with a KH reading as low as yours, altering the pH will be no easy task as you don't have a lot of buffering capacity. You have to be mindful of causing any large pH swings, which would be more harmful than having the low pH to begin with.
Since your test kit bottoms out at 6.0, I am wondering what your actual measurement is.