I run hydro and I have seen this before. Just a couple FACTS, not just throwing shit out there. In general, leaves curl up when they are lacking something, and curl down when you have to much of something. I wold rule out over nuting, becuase you don't have any burnt tips and the leaves would start to curl DOWN at the tip. It COULD be the start of a potasium deficiency as that will affect the dges of the leaves like that, if the tips on the edges start to brown then you can be sure that it is potasium def.
In the end, plant problems don't typically fall into one nice neat category. "ohh, I just need a little cal mag". Honestly to me it looks like you have harsh environmental conditions as the plant leaves look dry and haggered. I think your problems are the following:
1. Temps a little high.. high temps will cause the leaves to "taco" or start to fold in half, it will also make the plant tissue look dull and "dry" . 80 degrees 2" above means your leaf surface is probably 83-85. not ideal.
2. Your humidity is too low. 50% is great for late flower but in veg with young platns it is too low. yeah, you can do it..but we are talking about ideal condtions and your having problems soo... Your really beating on your plants with low humidity and high temp.. it creates a VERY high vapor pressure gradient. (just google it).
This excerbates the condition of too hot and causes all sorts of problems..chiefly too high a rate of respiration as th air outside the leaves is soo much drier than the plant tissue. SO when you ahve high heat and dry air, you are forcing the plant to transpire pretty much as fast as it can. When the plants are young and the roots are not developed enought to support the rate of water absorption and movement through the plant.. the leaves get dried out and they will move in order to reduce the rate of transperation. ERgo... the plant is drying out and will therefore close its stomata, effectively reducing the rate of transpiration and photosynthesis since Co2 is no longer absorbed. This typically corresponds to slower growth rates.
3. Your nute solution is a little too strong. (first off, for fuck sakes buy a PPM meter so you can actually tell us what the concentration of your solution is and actually learn what plants want and what happens when you give them what. A strong nutrent solution will make it more difficult for the plants to absorb water. Plants absorb water through active transport across the root membranes. The amount of energy it takes to absorb is dependant on the concentration gradient between roots and solution. Water travels from a lower to a higher salinity solution so your plant actually has to work to absorb water in the opposite direction (the roots have a lower concentration of nutrients than your solution). This contributes to the problems listed above.
So, If I were you...
1. keep the lights raised a little
2. lower your nute concentration (and probably back off the cal mag a bit. 5ml/gallon for young plants in hydro seems a bit excessive) that is what I add for week 4-6 of flower.
3. lower the temps..shoot for about 75-78 air temps
4. raise the humidity. Hang a wet towel, mist the plants, spill water on the floor... do something.
5. Foliar spray the plants with kelp extract... I'm tired of typing but it will help a whole host of things.
Whatever you do, WRITE IT DOWN and write down your observations as well. That is how you become a better grower because after several years of growing you will forget what caused what and how you fixed the problem.
