To me it looks like nutrient burn at the leaf tips, and you were right to flush the soil! I would flush your soil an additional time and remove a significant amount of runoff. If you are noticing improvement after this, I would increase the amount of runoff during future watering to continue to flush any salts built up around the roots, and also decrease the amount of nutrients given generally. The plants otherwise seem nice and healthy, and the grow is looking good! 👍