I see you put a lot of energy and time in your grow. But their is something that you are doing that is not right. I cannot pinpoint it. I have zero experience in DWC. Could be small things like pH pen calibration not saying this is it...) But a 8 week old DWC plant should be a small tree. Your seedling is dying. Best thing you should do is subscribe to a cannabis forum like grasscity, rollitup etc and ask for help.
I wish i will see in the future a growth of yours that is thriving and looking happy!! Best of luck!
Edit looking at ppm number closely I see major drop from day to day, That is your problem but it is hard for me to explain what is going on. Maybe too much supplement not enough food? (sensi grow) Ppm variations affects ph. Look at Advance Nutrients feeding chart and check if you are on range. It seems low for DWC-hydro.
Your total ppm would be fine with tap water and base nutrient only with coco coir growing medium. But with 5ml a gal this is good for first week of germination IMHO.
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