Leaf has dark spots/taco/end curl

[BlueBudz]

Hey people, kinda weird it’s only one leaf on one plant out of three in the same DWC. Strain is critical, clones are about 32 days from cutting. Been in the dwc since feb 26. 18-22°c res temp, humidity 45%, room 20-26°c. Target PH is 5.8-6 seen as low as 5.4 and high as6.5. Feeding DNF nutrients @half recommended strength. Using hygrozyme, cal mag, phdown. My water is from the tap, let it stand for a few days open first ph is 7-8-++ sometimes. I use a lot of ph down to get and keep water at proper PHlevels.They are growing very well tons of nodes and new green growth. Any ideas? I can take pics of the plants later when the lights come on.

[UrbZ]

Thats weird thats its only one leaf. Maybe the stem was hollowed out for some reason and that leaf didnt get nutes. Cannot see another reason why.

[BlueBudz]

Or it was the first to be affected. Hope it was an isolated incident but I fear more to come