Possible Iron, manganese, or sulfur deficiency?

[Legended]

I'm getting yellowing on the younger leaves, which I'm assuming is because I'm not balancing the pH of my tap water. Tap water usually has a pH of 7 -7.2 and that might locking out some nutrients. I ordered Apera pH pen, which should be coming in on Monday. I already have pH up / down, but I don't want to use it until I can take proper measurements with the pH pen. I want to drop the pH down to around 6.4 and add 5ml of CalMag per gallon, which I think would be ideal, but I would like an expert opinion, since this is my first grow and I lack the experience.

Here's a few pics of my girl:

I've been reading Marijuana Grower's Handbook, by Ed Rosenthal and here's a few pages, which brought me to this conclusion:

 

[Mañ'O'Green]

@Legended Welcome to AFN Kudos for doing your homework! They do look a little bit micro-nutrient deficient. Do not use cal-mag for this. Get some Earth Juice MicroBlast and do a foliar application. You can also use a Fulvic acid 5 parts to Kelp 2 parts mix. Do this just once.

It might also be helpful if you filled this out for us. There can be other causes for this appearance.

Please fill in this form: (copy and paste part is below, this first one tell what specifically to include)

-Problem: (brief description)
-Medium/grow method: soil; soilless-- coco, or peat based like Promix, etc. (please provide the actual product name); DWC, NFT, etc.
-Feed and supplements used: include brand, dosage/strength, frequency of feeding and watering (alone); method-- by hand, drippers, rercirc' or drain-to-waste,... N-P-K #'s too if you can!
-Water source: RO/DI; tap- dechlorinated-?..... EC or TDS reading; pH (don't bother with this on RO/DI, do bother with TDS/EC though to confirm it's working well enough)
-Strain and age
-Climate: night and day ambient T and RH%; res' temperatures; any extremes in T/RH% exposure
- Light used: HID, LED, COB, combo of,... wattage; light cycle hours (20/4, 18/6 , etc.); distance to tops....
-Additional info: PH in the root zone. How long have the plants been affected?...How fast did symptoms appear?... Anything else you think might be relevant..

--Pictures including WHOLE PLANT PICS, and troubled leaves.. use normal light or flash as other light sources ruin color rendering, critical for diagnostics! Turn off the grow lights.

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Problem:

Medium/grow method:

Feed: and supplements used:

water source:

PH in the root zone:

Strain/age:

light used:

Climate:

Additional info: