I use big rootz soil. I started them under the lights and moved them outside just recently. They started with this damage under the lights. It is getting worse , I’m afraid I’m going to lose my whole crop of 9 plants. The damage started on bottom now it’s going to the top. I have always measure the ph in the water , I get it to about 6.3 to 6.5 and use water that sits outside to get the chlorine out. I had them in red solo cups and the picture was then in those 3 gal pots just about 1 1/2 weeks. They are outside now full time and I love on New England the temp varies. I have brought them in if it was too hot or too cold.