Problems in DWC

I’m growing for the first time DWC ( this is my 2nd grow) and I run into some problems already.

I m growing from seed Super Lemon Haze (Greenhouse Company) in DWC and I’m on week 7 (day 46) from when I pop the seed in to the rockwool.

I use Low’s 5 gallons bucket as my reservoir.

I feed the plants Current Culture nutrients at : Coco Cal 2 ml/g

Veg A 2.5 ml/g

Veg B 2.5 ml/g

Silica Blast 2ml/g

SuperTrive 0.3m/g

UC Roots 2ml/g

Rapid Start 1m/g

Total PPM between 600-650

PH 5.8-6.2

My PPM’s are dropping from 600 to 500 and PH drift from 5.8 to 6.2 in less then 12 hours so I have to add back nutrients morning and night to bring it back to 600 PPM.

Recently I got into some problems, I got brown necrotic spots and wholes into the older leafs.

I will attach some pics

I will appreciate any help