HYDRO Cotyledons turning white-ish and curling down for no apparent reason

[ars]

So as my third or fourth grow, I thought I'd turn hydro because I'm just sick of the fungus gnat problem. I treated myself to a GHE Aquafarm (45 l), bought some special hydro nutrients, a PH and EC meter, and got started with Royal Hulkberry Automatic.

It is my second attempt to grow hydro now, I use (pretty alkaline, around 7-8) tap water as a basis and (organic) PH Down to keep the PH in check. The first time, I failed because I didn't check the PH regularly enough (once a week), the growing plant (only cotyledons + 2 leaves) just turned violet-ish and died off because the PH was simply too high.

This time around, I took care of everything: with tap water as a basis (+ nutrients so that I reached ~500ppm above the base value of the tap water), I check PH regularly and keep it around 6. Like last time, however, there are white-ish spots on my cotyledons and they seem to die off again? I suspect that the high lime content of the tap water may be the cause, but I'm not sure. Instead of just waiting for it to die off - does anyone have an idea what could be the issue here? See the photo below. Thanks in advance for any advice! I'm clueless since PH is fine, EC/ppm is fine, lighting and temperature as well...anyone else had similar experiences?

(Hope you get the idea, sorry for the bad picture. https://imgur.com/a/cteNeiM)

[YYCannabis]

If the first true leaves are ok, i wouldn’t worry too much about the cotyledon leaves dying off. The true leaves should be taking over the workload of feeding light to the tap root.

[Catfish22]

Try some spring water. I get mine at walmart about 65 cents a gallon. Ph is always right at 6.0.(but I check it anyway).
That will rule out your tap water.