Hi, I have been trying to grow lettuce indoors hydroponically but I find that the leaves develop yellow edges and brown spots before I can harvest. I am growing Tom Thumb lettuce in a deep water culture setup with 15 plants floating above a little over 4 gallons of nutrient solution with an air stone. The pH of the solution has consistently stayed between 6.2-6.4. Light is provided by a Taotronics TT-GL14 80*3W 6 Bands LED grow light hanging approximately 12” above the plants for 16 hours every day. I use coconut coir as my growing medium. For my nutrient solution, I followed instructions that I found online which called for a solution of 350 ppm (1 tsp/gal) MaxSea 3-20-20 kelp-based fertilizer while the seeds were germinating and until the seedlings grew four leaves, followed by a 700 ppm (2 tsp/gal) 16-16-16 MaxSea solution with 1/2 tsp/gal Epsom salts and 4 mL/gal Thrive Alive B1 once the seedlings were transplanted into the deep water culture setup. For the first deep water culture solution, I also added 1/2 tsp total mycorrhizae powder and a couple Mosquito Bits with Bacillus thuringiensis israelensis (to control against gnats and other potential pests). I changed the nutrient solution every two weeks. When I noticed the yellowing and browning in the first batch of lettuce, I tested the nutrient solution with a pH/TDS meter and found that the concentration was not anywhere near 700 ppm (taking into consideration the fact that the water itself as well as the Epsom salts and other supplements could influence the TDS) but in fact close to 1800 ppm. For the second lettuce planting I therefore tried to use the TDS meter to determine my own, hopefully more accurate fertilizer measurements, and settled on 1 tsp/gal 16-16-16 MaxSea with 3/8 tsp/gal Epsom salts for the 700 ppm nutrient solution, with the quantities of the other supplements staying the same. I seemed to have better results this time, but still encountered more yellow and brown areas on the lettuce leaves than I would like. I would greatly appreciate help narrowing down what could be the cause of the yellow edges and brown spots, or at least eliminating what couldn’t be the cause. I have attached a photo of the worst plant in the first batch, after it was picked approximately nine weeks after sprouting (I removed some of the older, yellower leaves from each plant in the first batch after six weeks when the problem first became noticeable), as well as photos of the second batch at four and six weeks after sprouting (the second batch was harvested at a little over six weeks). One possible explanation I recently thought of was age. Am I simply waiting too long to harvest my lettuce? Thank you for your help.