Is this an issue with test kit?

[Essjay]

So I just tested the water with a new liquid tester and it came out as 6dh for the tank and 4dh for the tap water.
I think that works out as 60 and 90 ppm?

4 dH = 72 ppm
6 dH = 107 ppm

There's a calculator on here for converting hardness units and it includes degrees Clarke and grains per gallon as well as dH and ppm (listed as mg/l CaCO3)

Calculator

 

[itiwhetu]

The test kit will never be the problem, manufacturers insure that.