dfk41 That liquid kit is the one. You would test for GH and KH. Considering we don’t know if re-mineralisation is built in, which I doubt is, you don’t want to add anything if the alkalinity is already in the 40 mg/l - 50 mg//l per 5 mg of tested water.
For example, a fresh osmio re-mineralisation kit would on an average have the following:
GH avg = 17-20 mg/l per 5 ml
KH avg = 45-55 mg/l per 5 ml
API GH & KH TEST KIT Freshwater Aquarium Water Test Kit 1-Count
The URL doesn’t work here on my handset.