This appears to be a nutrient toxicity. Since it’s starting at the top and I see your leafs are very dark green makes me think you have been over feeding.
I suggest you flush your media out with pH adjusted water at the rate of 1-2 gallons per gallon of soil. This will remove any build off of nutrients in the soil that have accumulated over time.
Remember when feeding you need to have about 20% run off out the bottom to wash away the old salts so they do not build up.
I hope this helps.