Please help !

Think overwatering with a possible ph issue.

I think Thinlizy is right. Also a great thing to kill gnats is pod poppers. They kill them flat out for me when they get to be a problem. https://www.amazon.ca/Nema-Globe-Beneficial-Nematodes-S-Feltiae/dp/B01N5TYSFU

Looks like you over watered. The plants drooped then you panic flushed and made things worse. In soil you never want it "WET". Only moist at all times. Let them dry out things will work out. You can also tell you have been over watering by all the gnats you have.

It's SOIL !!! BioBizz All Mix with max 10% BioBizz Worm Humus. That's all feeds I made from week 4 to week 7 now. I feed reraly , but heavy as a boost, mainly try to keep soil alive. I haven't flushed them, on week 4, I gave 800 ppm heavy feed boost. I feed very little and rare,mostly bacterial teas. They didn't loved it, bounced back, but I backed off for 3 full cycles before giving first tea even. I have two new things : 1. We had first frost outside and room temps dropped from 28-27 to 22-24 cel. 2. I increased average watering dose by one pint, they are huge, so increase is natural and I never met plants acting like this. My general idea is overwatering too, but they should be back by now. I gave them food at 12 o'clock from 18 yesterday, after 2 hours next day - they look like in photo.

Flush the girls with florakleen or any other product I'm Hydro grower but last week had the same issue/nightmare... Fear and Loathing in Las Vegas 😱🙈😅 Good luck with it 💪💪

If you gave them 800ppm and they did not like it, then bounced back at 600ppm and they did well. why would you go up to 900ppm? If the one least watered is doing well it points to too many nutes. Are you monitoring your EC as well? I feel if they are showing happy healthy signs at 600ppm keep it there for a bit and slowly increase by increments of 20 over a few days/week to 700 then creeping your way back to 800 as they grow and need more but paying attention to the signs they are giving you. If the leaves are drooping but the leaf stalks and stems are upright, that's a sign of over watering. Under watering leaf stalks and stems are also limp as well as the leaves. and coir is easy to over water if its not combined with perlite or other items in the medium. I'd recommend adding in a air stone to your reservoir, oxygenated water is good for them. Best of luck

With all the watering/flushing/feeding you are describing, maybe you are overwatering? What time was this taken? How much time before the lights turned out for the night? If you're hitting your plants too hard with light, they could be drooping like this a few hours before lights out.