probably a little too much water. let the soil dry out a little more before you water.
your leaves are droopy because they are heavy with water.
the yellowing looks more like a deficiency than fert burn. i could be wrong, but i think fert burn starts at the outside edge of the leaf and burns its way in.
if the light is burning the plants, that's different. you need to take the temperature of the air between the light and the plant - not just the temperature of the room because it will vary widely.
I would say your lights are too close to the plants, and you're giving them too much water. Well, maybe not too much water but too often.
Over-watering will also cause pest problems.
My suggestions:
Raise the lights a few inches.
Let the soil dry out a bit. Just watch the plants, not the soil and not the calendar. The plants will tell you when they need water. You'll be surprised at how dry the soil can get before the plants start showing ill effects.
Increase ventilation. Get a fan and point it at the trunks of the plants. You don't want air to stagnate down there under the canopy. Airflow will blow them gnats away.
