Help Identifying Problem: Light Burn?

[Triffid]

Gorilla Glue Auto
Week 6 of flower
DWC 35litre reservoir @ 19c
LED strips 3500k @ 750 - 800 PAR - 30cm from plant
Temps from 19c to 26c
RH from 40% to 60%
PH 5.5 to 6.2
AN nute line @ 1.2EC
Salicylic Acid, Chitosan and Great White myco additives
2x2 room

This plant got nuked with a Salycylic Acid overdose, which is where all the burnt tips and purple stems come from.
I have had some troubles with PH having massive overnight drops in the plants early days, think I had some bad bacteria in my res.
I cannot spot any pests or mold with a microscope.

I have had this pattern come up on my plants before, and thought it was Magnesium deficiency, I have been adding Mag/Cal, but still getting these burnt areas.

Is this light burn?

 

[PK]

@Pennywise is your hydro man

 

[Pennywise]

Looks like calcium deficency

 

[Pennywise]

Run your ph closer to 6.2 I’m guessing this is a ph issue. It’s also showing nute burn so may want to lower you ec a bit.

 

[Triffid]

Thankyou, that would make sense, started about week 3 of flower.

Started getting this in my grows after I moved, different water supply must be lacking in Calcium.
When someone points it out, seems obvious, many thanks, (great read in the links of your sig)

This is the first grow that I have had troubles with the PH, usually rock solid, but it's been plunging dramatically fairly consistently. I get a couple of stable days after a water change, then it starts dropping like a rock again.

I switched from an airstone to flooming, thinking the old airstone might have been responsible, but that didn't stop the PH plunge. Was thinking of switching the res to sterile (h2o2) for a week instead of adding beneficial bacteria.