4 week old ak48 leaves tip are yellowing

Please help asap my 4 week old ak48 plants the tips of the leaves are turning yellow and are turning up . growing in a dwc with sensi grow and super thrive is all i have gave them ph is 5.8 and around 270ppm.watering 3 times a day for 30 min each time.just put in a feeder tubes at top of plant but otherwise no changes to shock plant

Looks like nute burn, i dont do hydro 4 weeks is young, and some strains are more sensitive. Dilute alittle and see if the browning halts?