What deficiency is this?

[DankOfOrmy]

Hi so I've been growing 2 plants for around 3/4 weeks and they're doing really well however when I checked on them this morning on one of the leaves on one of the plants I noticed it wasn't right, I feel like it's a calcium deficiency however I am not to sure

 

[Scientific]

Hard to say. If you could answer the questions at the link below that would help a lot.

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