@fassette, I only used 0.5 ml/L BioBizz Fish-Mix during the vegetative stage (soil also from BioBizz, Light Mix). With my current grow of Purple Haze Auto, I also experienced that one of the plants was much slower and is, in fact, still way smaller than the other. As long as the plants look happy (yours seem green and healthy), I think it could be some phenotype variation or maybe a bad seed. I am still a beginner too, so this is just an educated guess.
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