you sure they just dont need more water? Are you running a drip system, dwc or? Why water 3 times a day? Hydroton pellets are airy enough that I would run a drip system 24/7 in hydro and never turn the water off.

It MAY be nuteburn....I'd cut things back but I'd also try running your watering schedule much more often and see if it helps.......if the plant looks better you can start putting your number back up......slowly.

Keep your ph between 5.6-6.2.......6.5 is considered the outside parameter so 6.7 is not ok.....

lots of possibilities..... you'll have to go one by one until you find out exactly what the issue is.....or even a combo of above symptoms together.

Good luck.