We need way more infor like what you are using to grow in, how often you are feeding, watering, what pH is, where on the plant this is taking place...
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the first pic is the fan leaves from the middle of the plant the second one is also the some the 3 pic is the top of my plant it stop growing and is extremely thin the last pic is the bottom of the plant. I'm using a bubble system my pH is 5.7 i am using GENERAL HYDROPONICS Flora Series my ppm level is 745
your ph is too low raise it to 6.2-5 ish. your plants must have a ph lockout and dont suck in nutes. they show defficiency by browning its not burn.
2ND grow ( current )
are your plants getting enouhg Calcium? it doesn't look like they are.