Hi, it could be the first signs of magnesium deficiency since it starts with the older and lower leaves, or it could also be nitrogen, which begins in the lower area, but I tend to lean more towards magnesium. I would recommend adding Calmag, and if it gets worse after that, you'll need to take other measures. - You can also measure your runoff. That's what comes out of the pot after watering. If you have an EC meter, the value shouldn't be over 2; otherwise, you have too much salt in the soil, which leads to blockage. You should keep the pH value between 5.8 and 6.5, so you can be sure it's a deficiency and not over-fertilization. If these values are correct and you give CalMag and it gets worse, feel free to reach out, and we can possibly say more. Good luck!