911/day 43 of 12/12

First grow ,im in 43 day of 12/12 ,LP spray hydro / water temp 68 deg /Ph 5.7 / EC .6 ( lite feeders never got over .7 EC entire grow)my water PPM is 16 on .5 scale and 400w hps. temps stable lights on 78 lights off 66-68 deg.

So here is what I noticed today 20 to 30% of leaf tips minor clawing and very end of tips yellowing (brittle dying) they are due for a res change(7th day) ,witch was the first res change that I did not add any veg nutes to the bloom sched I am following ,

but what I also changed in the past 24 hrs was I went back to 2 min on 7 min off water cycle,before that it has been pump continues for the last 5 wks, So I am wondering if this might be the issue or ,

what I am reading is a Potassium deficiency .

so you are 6 wks in. I would raise the ph a little. I like to run 5.8-6 from 6 wks on.