RE: my question, Thanks very much for the helpful replies everyone! :)
I guess I did indeed overdo it a bit with nutrients despite my caution. (At first I thought they were actually underfed due to corkscrewy/pale new growth) And at the beginning of week 5 I gave around 550-600 ppm (including the 200ppm water) because those plants were growing so fast.
Thankfully, it is mostly yellowing rather than necrosis.
(my tap water is 200-230 ppm (~0.4 EC), so that's a bit less than 400ppm (0.8 EC) of added nutrient. I always water the soil thoroughly until I see some runoff, and wait until the soil is very dry for the next watering)
Today the pots were "basically" flushed with 0.6EC (super small dose of NPK). I decided that was a stupid idea and just threw a lot of regular PH'd tapwater on top of it. I measured runoff at 220ppm. Unfortunately I did not test the initial runoff, but at least what came out in the end came out as clean as my tap water. :)
I'll proceed with the tips, give them some clean water for now and see how they will do π
Overall I am not very worried because they seem to be developing pretty well π
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