Feeding schedule.

hey guys. Ebb and flow hydro set up, watering 3 times a day/ 20 hr lights on MH 250w plants are doing well some huge leaves already the size of my hand only about 14 inches tall some of them roots coming out the sides of the rockwell. My question is if anybody thinks its too much time in between feedings 8Hours? i was thinking of cutting it down to every 4 hours instead, Or should i just keep it at 3 times a day plants are doing very as it is but i dont want to keep upping the nutes when maybe all is needed is another watering in between. Ph is at 5.7 nutes are at 900 ppm.

also i noticed what i think is some salt build up maybe on the tops of the cubes how do i get rid of this? just flush water over them?