Brown Spots Between Veins--HELP!

I have a variety of strains in 3 different mediums. All are experiencing the same problem which is brown spots between the leaf veins (see picture).

I use R/O water, and check PH before each watering. I use house and garden nutrients (coco and aqua flakes).

Any idea what could be causing this?

Detailed information below.

Outdoor Coco:
Bonticare Coco Coir
House and Garden Coco Line and supplements
Water every other day
PH 5.8

Outdoor Soil:
General Hydro expert line with supplements
PH 6.3
Water when needed

Indoor aeroflo2:
House and Garden aquaflakes and supplements (veg cycle [algen extract, magic green, root excelurator)
PH 5.8
No timer, running 24-7
H202 for first 3 days, then added root excelurator

use calmag to resolve this

^^True true

It is a magnesium deficiency.

thank you very much!!

When your ph or nutes are off you will get a magnesium deficiency, so something to think about

It showed Cal/Mag def .do u use any epsom salt or cal/mag plus?
I advice use one of way to feed them Mag or Cal...
R/o water poor in 2 nuts and the prb shown ....
What your ph level?

Let me know if I am doing anything incorrect:

I typically calibrate my PH meter once a week. I use a Hannah 9813-6
I use a stealth R/O 100. ADMITTEDLY I haven't changed the filter in 5 MONTHS. Could this be contributing to the problem?

I PH my coco feed to 5.8 before hand watering (drain to waste). I PH my Soil feed to 6.3 before hand watering (drain to waste). I try and keep my aeroponics system reservoir to 5.8 (sometimes I am forced to let it float from 5.5 to 6.1)

Do I add Cal/mag (General Hydropoincs..picked up at local hydro store) directly to the nutes soup, or should I treat with a heavy dose alone, or FOLIAR (what dose)?

Thanks to everyone for such a terrific response. This forum rocks!!

I have not been using Cal/Mag in the past, as I have never experienced this problem. The weather is colder, maybe that could be a contributing factor as well?

I will begin to supplement with Cal/Mag. My plan of attack is to add a light dose (1 tsp per gallon according to General Hydroponics label), adjust ph to 5.5, then add house and garden nutes according to instructions.
Is this the proper way of supplementing R/O water with Cal/Mag?
Does my doseage sound correct?
Will a filter change on my Stealth R/O unit make a difference (It doesn't appear dirty at all).