Looks like nutrient burn. Your grow mix in too hot for seedlings at 12-14-24. You should have use this mix in the lower half of the pot then used a light mix for the top layer. Some growers will blend a lighter mix into the hotter mix to lower the N-P-K ratio. You can try to flush the pots to get the ppm lower but you will create a new problem of having a wet root zone and will have to stop watering for a while.