Dark edges of a few fan leaves

I am growing three auto flowers. This is my first grow. I noticed some darkening of edges of a few fan leaves . I use the Fox farm trio in living soil. Growing indoors with LED barrina 2000. Flowers started appearing a few days ago . Anyone have any clues what this is? Online I looked at a few images and was wondering if there's a phosphorus deficiency or phosphorus or a calcium deficiency . I gave sensi Cal-Mag one time about a week and a half ago. Anyone have any clues? Plants look mostly healthy, just trying to maximize everything. Water pH is about seven maybe 7.2. I just ordered pH down.
 

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