Hi mate, how many leaves have the problem, it dont look like you have brown spots or burned patches on the leaves, could it be that you have spilt something on the plants or have you been misting the plants with the lights on, sometimes you get patches like that if 2 leaves have been lying on top of each other,
the plants dont look like they need nutrients so just use plant water for a week or 2 or until the plants look like they need more, some products your suppose to feed every time you water the plants and others you dont, their is a nutrient feeding guide in my signature so have a look in their.
remind me what product your using and at what dose,
it dont look to bad to me, it could just be that your misting the plants or the leaves have been touching each other, but it certainly dont look like they need nutrients so id just use plain water for the next week to 10 days and see how they look, if the leaves start to look a lighter shade of green then you know they need nutrients.
as im sure you read in my journal im getting amazing results but not watering as often and not watering as much when i water, this has meant i have not needed to add any nutrients at all to some of my plants, im now under 12-12 and wont feed nutrients on the test plants until i can see they need nutrients, so far their just growing like crazy and even after doing a defol on a couple of them they didnt seem to slow down with growth, so im thinking that the lack of water is forcing the roots to grow a lot quicker so they can reach the water, then when i water the plants the growth is very aggresive for the first week after watering, my plants have reached flowering height much quicker and im not using as much nutrients on my other plants either,
the nutrients im currently using your suppose to give nutrients every time you water the plants, but im going every 3rd feed with nutrients, so i give nutrients, then water, then water again, then back to nutrients,
your plants dont look that bad, how many leaves have for that mark on them, if your misting them stop misting as the plants dont need misting, i do mist mine but only once a month and i do this just to wash any dust off of them, but i mist the plants then i turn my big fan on the highest setting so it then evaps the water a lot quicker and the light then dont burn the leaves
so remind me what nutrients your using and what feeding schedule your using and how often, then let us know how many leaves and plants are affected, if its more than one plant that its clearly a nutrient problem or misting problem, if its just on the leaves that are touching other leaves on other plants then these will swet and cause pm problems, so i remove any leaves that are resting on each other as the last thing you want is mold forming,
their is also a plant accident room in my signature as well, that covers pretty much every problem and tells you how to get it sorted, their is also a beginners guide to breeding and the nutrient feed guide is pretty good as well,
so let us know as much as you can then we can have a think of what it is and give you some idea of whats caused it and whats going to stop it getting worse, but if the plants dont look like they need nutrients then id just use plain water for the next 2 or 3 waterings and that will then rule out nutrients being the problem