Mg deficiency?

You have a magnesium deficiency, calmag and you're golden

The white discolored on the leafs can be mag deficiency

Usually nitrogen deficiency..but they kinda look healthy rn

Side note your humidity is very low as well and I would also consider raising your temp as well for the stage of growth you're in you want your RH to be in the high 70 along with your temp as well your VPD is very important for proper nutirent uptake along with water uptake as well...This could be why you're having a bit of issues with magnesium deficiency

I agree with EZZjayBruh, definitely the begging signs of magnesium deficiency and I would use a bit of Epson 1tsp per 3.6L and it to your normal watering schedual and you can do that by weekly until there is change

First off that 100W light is not hurting things at 15”.. The specs from manufacturer (which are likely inflated if anything ) show at 14” your peak PPFD in the center at 750 umols/m2/sec , with an average across the canopy being more like 600ish likely. To answer the question about mag deficiency. Yes you have SIGNS of it, with the interveinal chlorosis beginning slightly. That is a common symptom of magnesium deficiency.

secondo me si' ma è proprio al'inizio correggi con un po di cal mag e dovresti essere apposto

Hi there, nice grow. you're pH looks good but your light is close being 15 inches if at 100% power per your diary. Try taking it to 20 inches.