HELP! Yellow bottom leaves and brown spots

Help! Have three plants growing under a 400w hps, they are 3 weeks old and have been doing great.

In the last days i have noticed slower growth, some new leaves were born all deformed, yellow lower leaves with brown spots, tacoed leaves. This change has been subtle didnt look like a problem until today i noticed the brown spots.

I am using a soil mix, on the bag it says NPK Fertilizer (12-14-24) 0,75 and Organic Fertilizer (6-7-6) 6,5.

I thought the soil had enough nutrients for the first weeks..my guess is that the nutes in the soil ran out and now the plants are begging for them again, nutrient deficiency maybe?

Some pics
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help, going to the store tomorrow need to know what to get