SOS for seedlings

Hey all on my second grow. On my first and this grow after around 10 -14 days this has happened on Both occasions. Ive tried doing research and found possibly root rot or calcium deficiency is the problem based on photos I've seen. Im running canna a&b , cannazym and rhizotonic all at half dose, calmag at 1ml per 10l as suggested ph 5.5 in coco with 600w led 25c temp with a good airflow and carb bag inside. Seedlings are not leggy at all and up until today (day 10) they were looking great. Within the last 6 hours this has appeared on the leaves and now I'm worried about loosing the plants or stunting beyond salvage. Any ideas from past experience appreciated they were germinated on 2nd june and planted on 4th and have been flying since then until today.

 

Hi im they are in cup size pots with coco the. Bottom 1/2 inch is clay balls to aid drainage. Im feeding once top 1 inch is dry around once every 1.5 to 2 days allowing run off of about 1/3. I live in a hard water area where ppm is 450 from the tap. Yesterday i measured my ppm from the garden water butt at 47ppm but there was larve swimming around in it so scrapped that and im off to get some de ionised water which i plan to ph then flush them with. I did pre water the coco with canna a&b but they have been in the coco for 10 days and only displayed these signs yesterday rapidly. I plan to get de ironised water and start fresh and ditch my feed batch i have because its clearly freaked them out. Just wondering if anyone has had these leaf symptoms and nailed the diagnosis. Although not completely devastating and a learning curve it would be nice to have some guidance too. When flushing with deionised water as im in coco shall i just flush and not feed or flush and feed a light mix? Thanks