Just curious how far away from the tops you have the light? If it's closer than 2' you may be getting some burn stress.
Concerning watering, I soak mine, base and all, until entire soil is saturated. I then let it dry almost completely (you see the leaves slightly wilt) and re-water with nutrients. Those look to me like they could handle 250ppm no problem. What kind of nutrients are you using to supplement soil if any?
I so wish I could give out -rep.
2 feet? Really homie?
Let us discuss why 24 inches is too far to keep a 400w bulb from a plant.
A 400w bulb provides - at best - 45,000 lumens at 6.8 inches from the center of the bulb. Most 400w bulbs, with just a few hundred hours of use, are usually around 40,000 lumens. Every 6.78 inches from the bulb you take a square of the value of the distance. If 6.78 is One, then 13.56 is Two, 20.34 inches is Three and 27.12 inches is Four.
So at One: 40,000/(1x1)=40,000
At Two: 40,000/(2x2)=10,000
At Three: 40,000/(3x3)=4444.44
At Four: 40,000/(4x4)=2500
Beneath 2500 lumens Photosynthetic action basically grinds to a halt. So the top of the plant might get just barely enough light, but an 18 inch plant means that halfway down the plant you're looking at less than 1,500 lumens; less than 1/25th of the energy.
A 400w bulb is best kept within 24 inches from the bottommost part of the plant you are trying to develop. An 18 inch plant would benefit from having the bulb within 14 inches of the canopy top. If the bulb can be air cooled, the closer the better.