I think it's overwatering.. do you water a lot?

Go with Roberts, he could be on the right track and knows his stuff.

Judging by the plants your ph is likely wrong. What is ph run off? What substrate are you using? If using coco you need cal mag in your nutrition without a doubt. How warm is tent. 82 to 83 is ideal without co2. Can be more with. In your case I would focus on getting basics down before adding advanced growing techniques. How much nutrition are you giving them? How much water. Again if in coco it needs to stay 90 to 100 percent saturated. Very hard to overwater in coco unless you leave it under water. Feel free to contact me if I am in the ball park in pm for more help