Brown spot

I'm in the same boat as the others, I don't think its anything to worry about. If its progressing to other leaves then its something to look into. Good Luck!

I wouldn’t be too concerned with the brown spot- could be where you’ve accidentally spilled some fertilized water on that leaf. I would be more concerned with how small the plant is for being 3 weeks old already. Might be a genetic issue but most likely a over fertilization issue from what I can see. You are feeding an awfully high dose of fertilizer to an extremely young, extremely undersized plant. If you are starting with a high quality soil such as Fox Farms, there is no need for ANY additional food for the first 4-6 weeks. Just plain, PH’d water and your plants will thrive like never before! When I use soil from Fox Farms, I dilute the mixture 50/50 with regular potting/garden soil (unfertilized). The Fox Farms soils are usually too "hot" out of the bag but diluting them makes for the perfect balance. You are using a super “hot" soil in conjunction with a grow & bloom fertilizer as well as CalMag. Less = More when it comes to feeding Cannabis and your plant is trying to tell you that it needs a break. Cut off the fertilizer for now, flush the medium and hope that your lovely little lady can fight her way back to healthy. Sorry if this seems harsh but I’m only trying to help. My tip of the week in my current diary is all about this exact issue so check it out if you’re looking for more info… all the best growing forward and remember- Less = More! 🇨🇦