Whats wrong with my plant?

Hey everybody

Please excuse my bad english. One of my 5 plants (the other ones are healthy) has brwon spots on its leaves. Some stats:
-Flowering day 25
-using the whole biobizz line including 0.3ml/l biobizz calmag everytime I water using the manufacturers recomendations
-400w ndl 14inch away and 4x10w led‘s 8inch away
-biobizz light mix soil in 4.3 gal root pouches
-watering 0.64gal every 3 days/if the first few inches of the soil are dry
-82.4 F temperature
-45-50% humidity

thanks for your help

Up your Ca/Mg Additive, it's the color of a Ca deficit with the pattern of an Mg deficit.

thanks a lot, I will do that.

PH is around 6 which seems pretty low. biobizz states that its nutrients autocorrect the PH to 6.5. What do you think?