Lock out problem in rockwool

I hand water once a day in 6x6x6 rockwool at 5.8 1250ppm in week 4 flower. With recent grow, leaves are yellowing out and now has big brown spots. PH runoff is coming out at 5.4 even when trying to water at 6.2. PPM run off is 1450ish. Should I flush with tap and start new next day?
Hmm.im not much help.i only use 1" cubes to start everything..then it's straight to water...but it sounds like salts building up...sounds like needs a flush,but I don't know how to do that in that big of rw... maybe it's like that Coco stuff and you can flush?
If you are getting buildup in rockwool your aren't watering enough. Finishing a 6in cube grow hand watering in my eyes is not an option unless you are in your flower room for all 12 hrs.