Yellow droopy curling leaves on seedling

First time posting here, really needing some opinions and help.
My little plant is about 22 days old, sprouted in rockwool and placed in a dwc system with 4 1/2 gallons of water. I soaked the rockwool for about 5 hours in ph 5.5 which I've read could have not been long enough. I've been having ph issues since putting her into my deep water culture system, the ph was rising from 5.8 to 7.4 within a few hours at the start. Was having brown icky stuff in the res so started adding 12ml h2o2 which cleared that up. Now the ph will rise to about 6.6 in the same time frame. Can't figure out for the life of me how to keep the ph down. I am adding 3/4 strength nutrients at the moment, possibly thinking she was having a nitrogen definceny due to yellowing of the lower leaves. Adding more nutes has not seemed to help. Currently I'm adding 3 tsp of each floraGrow, floraBloom, and floraMicro. 4tsp calmag, 12ml h2o2. Is this too much for my small girl? Trying to keep ph at 5.8 but struggling. Also I just ordered an air cooled hood and exhaust fan, my tent temp when opened is about 85-90° currently, so hoping to bring that down with my new fan/hood. I'm using a 600w metal halide about 21 inches away from the top of the plant. The res temp has gotten up to 90° before, so I have thrown a frozen water bottle in there to try and keep that from being so hot as well. I'm at a loss here, not sure what to do for my girl she is looking very sad at the moment. Any help is very much appreciated. If more info is needed i can give more. Thanks in advance!!!

If you can't keep DWC water tempertures to a max of about 65'F than you shouldn't grow in DWC wherever you are, or you need expensive water chiller, this is probably where the brown shit came from and you should insulate the buckets with some kind of 'mylar blanket/bubble wrap for home water heaters' to keep things stable. Light and heat into the bucket is a killer. Water holds more oxygen at lower temp, you cant let temp of water get very warm, no matter how much air you pump in there the water will just not hold it at higher temps. The more air the better. the cooler the better. When first mixing up the DWC nutrient solution, it is normal for there to be swings and fluctuations in pH when first mixing the solution. Yes this can and should be pH's back down to 5.4-5.5 but it's not good to add too much pH and swings while they're growing from pH5.4 to 6.5ish is fine. Different minerals uptake at different pH values so this ensures the plant gets a full diet.

Rockwool should be soaked in pH 4.5 range water the initial soaking, then that water thrown away, and then first watering of the cube should be flushed through with pH 5.4-5.6 1/4 to 1/2 nutrient solution and that waste water thrown away. Now the cube is stable. If you have only a small little rooting cube in there, this makes no difference and wont cause any issue if you didn't do it this way, but if you have a 3 or 4" cubes under those rocks it may. Also, did you wash/rinse the hydroton rocks before using?

Lift the light higher in this time of stress for them. and they hardly need any food, just low strength and balanced, in DWC nutrient uptake is fast and emediate, take it easy on them.

If you've already added this and that and tried all kinds of things and added pH way to many times then I would throw that water away and start again, and let the water bubble for several hours before even attempting to pH it.

I think you're fiddling with them too much. Sorry I have no specific fix for you just a lot of jibba jabba info talk, a sorta checklist of things for you.

They kind of look too wet, like you have the water level up high with a rockwool cube submerged 24/7, which it shouldn't be.

I also ordered things for an ebb and flow, going to try that technique when my stuff comes in, I've never ventured away from dwc and I feel it's about time I try my hand at a different system. I've always had success with my dwc buckets though, this is the first time I've had so many issues. It's rooted in a 1.5" rockwool cube. I rinsed the hydroton as much as I could, definitely until it ran clean and even past that. I've heard it should be ph'd but also heard that it's ph neutral so that would be unecesaary? I just rinsed the hell out of it until the water was clean. I was actually thinking it looked overwatered as well, i'm going to take some water out and raise the light some. I just changed the res yesterday so haven't messed with it too much besides ph'ing it down when I woke up. Should I be adding only 1/2 strength nutes though instead of 3/4? I was thinking it might be too much but my partner disagrees. Thanks for the reply!!

for DWC, yes, I would drop the EC value for that size plant it it's time of stress, feeding and photosynthesis have basically halted if you know what I mean. in a sense, we want to ease it's burden's in this time of stress, from light and nutrients to give it time to recover. Lowering water level is probably good idea if the cube and rocks up high near the roots are staying saturated.

Ebb and Flow is definitely more forgiving. Good luck man.

(And rinsing the hydrotonm until it comes out clean is the way to go, and just need to have the water pH you do that with somewhere around what you plan to use, don't need to be perfect, but it helps to always have anything that will be near roots nicely set to good pH range.)

Don’t mean to hijack a post but I see similar things happening to my seedlings (I’m in The Coast of Maine soil though). Seedling is Purple Afghan Kush. Under 4’ 4 lamp t5 raised about 2ft above plant. Just transplanted yesterday wondering if it just needed some good food. (This is not an over watering problem.) the other picture is one that I haven’t transplanted due to it’s not nearly as big (germinated and planted at the same time. Same strain
I’ve been growing for about 6 years but honestly haven’t done a bunch of seeds in that time and now that I’m going to be popping some more serious/expensive seeds I need to get it down. Any advice out there? Very much appreciated!

Mine was Afghani actually. After getting through the seeding stage it really did well, still not sure what the issue was though...sadly turned out to be a male..here’s a pic of right before I chopped em down. Sad day lol.