Crisp Leaves in DWC setup

I have 4 plants in an 18 gallon tote dwc set up with 3 airstones. I've been using GH FloraNova @ 300ish ppm and my plant leaves still managed to get burnt. The plants are approaching 4 nodes so I thought that they'd be mature enough to handle more than a few hundred ppm but I was wrong. Any suggestions on what to do from here? The plant's lower leaves had brown veins and started getting crisp to the touch.

well if you say the plants have nute burn then I would change out the res. and start over and see what happens,you didnt go into detail on your grow as to your ph level what type of water(tap?) how much light res temps grow area temp etc.

Yup, from what you said, it just sounds like you need to lower your nutes.