It appears everyone has told you right. If that issue is showing on just one plant like that, it could be a genetic anomaly that caused it or it could be that some water formed droplets on that leaf and then the light burned the leaf beneath because of the magnifying effect off the water under the light. The good thing is that the plants look healthy otherwise and are steadily growing. Once they get bigger, they will indeed drop those early leaves that get the plants started. That damage will be gone with the leaf once it is used up by the plant.
On feeding, I use a rule for seedlings. Watch the little ffirst leaves that are tiny and round. These are cotyledons (seed leaves) that are actually not true leaves but rather are the food source ffor the seedlings while they get their roots established. When you see those little leaves begin to yellow, it means they are being used up and its time to begin feeding. Like DGF said, begin feeding at 1/4 strength nutrients for the first week, then increase the nutes each week until you reach full strength after several weeks (1/4 then 1/2 then 3/4 then full strength).
The Hemp Goddess touched on something critically important for hydro growers (and all MJ growers alike) pH is the life or death of your plants. It is one of the main points to control ffor hydro growers and is second only to light and water. I don't know how much you know but for growing in hydro, you must maintain a consistent pH in your water and that means regular monitoring and adjusting of the pH in your system. Iff you don't have a pH tester then you need to get one right away.
Once the plants begin feeding, they need to have a pH around 5.8 in order to be able to take in nutrients through the roots.
I recommend that you learn how the pH changes over time within the reservoir because it will change as the plants take in nutrients and water. I have found that in hydro the pH can and will "drift" from high pH to low or vice-versa. What you want to do is mix your nutrients in your water then check and adjust the pH to around 5.8 and then monitor it over the next few days. The plants in hydro will do ok with the pH drifting in the range between 5.3-6.2 but if it goes beyond one of those 2 numbers, its time to reset the pH with either pH adjuster or new water/nutrient solution. You also don't want to allow the pH to get close to the edge of the allowable pH range and hover. If it gets close to one end and hovers for a day then change out the water with new water/solution. If it moves quickly from the starting point to one end then reset it with pH adjuster. If it does the same thing again, then you need to add more nutrients because the plants are stripping it out of the water. That is a good thing as it means the plants are eating and are ready for more ffood.
