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After a few days this shit began to appear on 1 plant. Only a few other plants have spots but none like this one. Its pretty localized to that plant. I think it might be an insect problem but I can't find any insects or larva. I took the ones out of the soil and thought I saw a few thripes. I also say some flying thripes about 5 days ago and killed them all. The ppms in the res are now at 60ppm in tub 1 and 40 in tub 2. The plants otherwise look healthy. the large one shows no signs of spots or necrotic spots so I know they are getting enough calcium and nutes. The large one however seen in the picture has some dark edges on the lowest most leaves.

I have a Hot Shot Strip, the one OpieYutts suggests. What should I do?

The ph is at 5.8 for both so no problem there