Seedlings dying need help with diagnosing

Hello GS community,

I have been here reading / sucking information for the past 5 months. Already growth a batch using the information here (thanks to you all)

I had a 60*100 cm wide, 120 cm height grow cabinet for my first grow and i got some pretty decent yield using bagseeds under 250w hps for the whole growth.

So this is my second grow. Now i built a second grow cabinet which is 40*70*70 cm in size for starting seedlings and first stage of veg growth and then i'll use the bigger cabinet for further vegetative growth and flowering.

Let me tell you about my grow space & conditions first to help diagnosing.

- I have 2 x 100 m3/h bathroom fans running. One for exhaust one for circulating air inside the box. Got passive intake.
- Temps are around 75-85 F (lights on 24/0) and humidity is around %50
- I have 12 23W CFLs for lighting but for now i am just using 5 of them. 2 for the now seeds that will sprout soon (hopefully) and 3 for the remaining 2 seedlings (1,5 for each so to say )
- Using tap water for watering pH around 6.8-7.3.
- Got the HESI COCO set but for now i am not using any nutrients yet.
- The growth medium is rockwool (used alone until they sprout) inserted inside a mix of %50 coco - %30 perlite - %15 vermiculite - %5 leonardite - 1 tspn of dolomite lime (as i said no nutrients yet) in 500 mL pots.


My problem is the leaves of the seedlings curling down almost forming C shape as you'll see in the pictures below.
I have planted 12 seeds and 10 (!) has died along the way. All showing same symptoms and dying by completely drying up.
I checked the root system just before i inserted the rockwool inside the medium in pots and they were looking fine. a lot of root tips (like 7-8 of them) were coming out of sides and bottom of the rocwool (mostly bottom)


It's been watered 2 times in their new pots. First was 2 days ago when i first inserted them into pots and today (surface was fully dried and pots were very light) So one thing i am pretty sure is that i am not overwatering. But i don't know what is the issue. Since they are not stretching i can also tell the lighting is just enough for now. But what causes this curling/yellowing ? Even though it looks like heat stress to me, the temperature is quite normal even on the seedling level. So what? please help me out...



In the 4th photo u can see that i started 12 more seeds in a self made humidity dome since these babies don't seem to make it...

 

I can't afford a pH-meter right now and i don't want to buy those cheap pH test kits. Our water company states that their water pH is 7.34. So do you think adding a few drops of vinegar (%4-%5 dilute) would do the trick? And i'll keep in mind to let the tap water sit a few days before i use them for waterig. Thanks for the quick response btw.

 

If that's the case, i think i have no other option left.