Hi all, Yes it should be, because your source of hardness is from dissolved limestone CaCO3 which is usual in the UK. You've added 1:1 Ca (measured as dGH) and CO3 (measured as dKH).
If you added calcium chloride (CaCl2.6H2O) it would raise dGH, but not dKH, and potassium bicabonate (KHCO3) would raises dKH, but not dGH.You can measure the TDS in the tank water easily using an electronic conductivity meter, it doesn't tell you wnich ions you have, but it tells you the total amount.
Conductivity meters sold for hydroponics often measure up to 19,000 microS, but you need a low range meter that ranges up to 1999 microS. If you set the measurement unit to "ppm TDS", the machine multiplies the conductivity value by 0.64.
The reason for measuring conductivity is that you have a tap supply which varies through the year (from the report you had minimum amd maximum conductivity values of 160 microS and 405 microS), but only one hardness value "5.7", presumably an average. Because your conductivity value changed through the year it is likely that sometimes the hardness value in your tap water was higher than 5.7 and sometimes it was lower.
I use rain-water in the tanks, this is also variable, higher conductivity/dGH/dKH in the summer (when it doesn't rain as much and it is dusty) and lower in the winter. The lowest rain-water value I've ever measured was ~30 microS and the highest ~150microS. I have a tap supply that comes from a deep limestone aquifer and the conductivity is always in the range 650 - 750 microS, and the hardness somewhere around 17dKH, it only changes a very small amount through the year.
I know now to add a dash of tap water (adds TDS, dGH & dKH) to the water changes in the winter, and a dash of RO water in the summer.
cheers Darrel