Several in one

Wendy told me there are meters that can test several aspects of the water quality. But you have to look carefully it's not a very big hassle to change the heads or calibrate every time. If the calibrating costs as much time as the druppeltest it's still a pain in the ass... We also have to buy stuff for the one you linked to, soaking solution, calibration solution and distilled water. But maybe that's for all of them, I don't know. Well, to conclude my story, I would read the manual before buying one... We have a document about the biology of aquaponics: