I never got to this in my video because my approach radically changed since writing the script. It didn't exactly matter for the video itself, but I think it's a good advice.
Originally I thought that beginner
#Kratky #hydroponics gardeners like me don't need to bother about EC, as leafy greens are tolerant (in my one-season experience, tomatoes are tolerant too). But should make effort to lower solution's pH to lettuce levels (around 5.8). Which is a struggle as I found pH meters to be drama queens, while my EC meter is more reliable.
That was before I studied recommended EC/pH tables a lot, and also ran an experiment on my surviving kale, and then learned that it applies to all brassicas: they actually _like_ higher pH, and also like much higher EC, so if you have 'a bit hard' water like mine, you can ignore pH, scale the nutrients up to reach EC 2-2.5 (using a reliable non-fussy device to check), and be a happy owner of a garden with kale, pak-choy, mizuna, mustard greens, and a number of other 'cabbages'.
I'm currently unsure why the 'hydroponic beginner default' is lettuce, while it's so much easier to grow brassicas.
This year's young mizuna and pak-choy say hello (with sorrel in the front bottle).