Need some advice on my tortoise food.

I moved my plants (grass, dandelion escarole) to my flood table a while ago. This is my first experience with hydro and I could use some advice.

A few of my plants are getting brown spots (see pics) and I'm not sure what is going on. I'm using master blend according to the website calculator, adjusting ph to about 6, changing nutrients about every 2 weeks (ro water). Everything is growing like crazy, just not sure about the brown-ness.

Thanks!