Help - yellow and curling

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1. Soil or Hydro? - coco soiless mix but growing like soil in pots
2. ferts/nutes used? Soil mix? - Fox Farm Grow Big 6-4-4 - used very little (see below); watering about every other day as needed
3. Lighting: Florous, HPS, MH, or mix? wattages? - Single 600W gravita light w MH bulb
4. PH if known? - water and water/fertilizer mix kept at 6.5 to 7
5. What stage of growth? - early veg, after 3 to 4 weeks from planting seeds
6. Relative humidity and temp of area if known? - RH = 40% to 50% (usually right in the middle); TEMP - lights on - 75 F away from light, and 77-79 F on floor directly below light; lights off - 69 F to 72 F
7. Grow area size and ventilation? - room is 8' x 8' x 8'; have 19 plants in small pots in approx 3.5' x 3.5' area. Have a 700 CFM exhaust fan high on one wall, and another 700 CFM fan at floor level on another wall; a rotating fan at 3' above plants and a humidifier to keep RH above 40% (goes to about 38% without it).
8. lighting schedule? 18 on (starting at 6:30pm), 6 off (from 12:30p to 6:30p); light kept at 3 ft above top of plants

Hi. I will try to keep this short, but I need help. I planted seed 3 to 4 weeks ago and my plants are short, growing slow and the leaves are yellowing. What can I do? What could be wrong? Any ideas or suggestions would be really appreciated.

As background, here's what I've done. I planted 30 seeds - a mix of sativa and indica - in a tray of coco cubes, below the MH light, but set light well above plants - maybe 5 feet to avoid heat burn. I watered only about every other day. 19 grew to reasonable seedlings and rooted in about 2-1/2 weeks so I transplanted. About three days prior to transplanting I decided to fertilize for first time with Grow Big, following label - 1 tsp per gallon. I saw yellowing and curling within a day, especially on two plants.

I flushed with water for two days, assuming I had nutrient burn. On third day I transplanted to the small pots, with loose coco mix, and watered again (no fertilizer). Since then the plants seem to have taken ok - all are growing and pointing straight up, but the leaves of all plants seem light, yellow/green, and the two most yellow before now have brown, curled top leaves. Some of the other plants are curling a bit too. Some look full but still light green/yellow. The tallest is about 3 to 4" and smallest about 1-1/2" and all look like they're barely more than seedlings.

FYI, I use a little vinegar to lower water and water/fertilizer mix pH to below 7. Don't know if this is a problem too.

Please, any suggestions would be welcome. THANKS.
vinegar is a temporary ph fix. I'm not familiar with cocoa fiber. What is the ph of the coco?.. your 6.5-7 is perfect.
"Soiless"...I don't believe coco has any nutritional value. So, I would think that they are hungry for some nitrogen. But I don't see a problem with your fert mix.

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