A lot of times when growing in soil you can develop hotspots from not watering the soil correctly. If you burned them that bad, I would suggest a real flush which would take 60-80 liters of water to clear the soil. When you water, ensure you have a good runoff coming from the bottom of your pots. 2 liters of water in an 18 liter container is not enough. The nutes will set up high in the soil and when you gave it a "good flush" you did nothing but push the nutes sitting high in the pot straight to the bottom and cooked the plant.