Little brown spot and leaves become yellow

Thanks for linking your diary! If you are using lots of coco coir in your mix, you will have to give extra calcium and magnesium (cal/mag supplement) about once a week. Also feeding more frequently than only once in 3 weeks, as perlite, clay pebbles and coir all have zero nutrients, maybe at this stage, every 5-7 days with half strength nutrients with a splash of plain water between feedings. There is no need to "rinse" the substrate at this stage/age, leave this until you get close to harvesting the mature flowers. Also, I would strongly urge you to go to 18/6, this is far healthier for your plants, all plants NEED sleep to finish their photosynthesis cycle and to have a correctly functioning metabolism. A plant with an incorrectly functioning metabolism will never grow to its best potential. In reality, the myth that it is ok to grow for weeks/months without sleep (24/0) needs to be removed from the cannabis world! You will not lose anything by going to 18/6, in fact you will gain much due to having plants with correctly functioning metabolisms. Nothing on planet earth can survive and fuction properly without sleep or a "circadian rhythm" of activity/rest/activity/rest..... 18/6 has been proven to be the optimum lighting schedule for cannabis through 90+ years of research and is not a random number. At this stage your plants are probably exhausted and under some stress. Anyway, feed a bit more often, add some cal/mag and go to 18/6 and in 2 weeks you should be very happy with how things have progressed! Hope this helps, Organoman.