They cover two ranges. If your water is either acidic or alkaline, you only need to use one. But, many find that their water is somewhere near the 7.4-7.6 area where the ranges overlap so it's difficult to tell exactly what you have without using both. My water is 7.5, for example, and slightly hard. Good conditions for a lot of fish and doesn't shift pH easily like soft water does. I use both tests every time, though, or I might not realize if it did shift a bit one way or the other.