Small veins in leaves yellowing mid/late flower, Pictures inside

Many thanks for taking the time to read my post and for any help.

Pictures at bottom:
Borrowed a questionnaire from another site for more info on my situation: at bottom:

I have been noticing for the past 3-4 days a progression of the yellowing of the small veins in the leaves at the top of this one plant, there is also, what i think to be nutrient burn on the tips and blade tips. This started on the PK boost week and i also pumped up base nutes to high also.. 1000 - 1100ppm YES BAD I KNOW, im a noob...this is why i think this is hopefully only nute burn.

This plant is 47 days into flower and has a flowering time of 60 -80 days depending on how high your noob factor is( mine is off the scale)....

This plant is in Straight canna coco being fed on full canna coco range(AB + Adds) and has been being fed as per cannas semi-customisable sched you can make up on their site.

This plant is in a pot that is wayyyy to small that what it should have been in( major noob mistake, i know) so too high ppms and not enough moisture will cause this plant major probs.. just a thought

The other plants in this tent, a week behind, have tip burn but luckly not progressed to the stage of said plant. Maybe i got the flush and feed reduction in on them justt in time

I have since done a large volume, low ppm(400ppm) flush and reduced feed ppm's to 500-550

Its still growing very fast, blowing up nicly (for bagseed anyway's) so lucky so far,

All things aside this plant and its sisters in my tent have been looking and growing fantastic in the canna coco + canna adds, but im now running into a returning problem: Major note: I also saw this exact problem on another plant i flowered of this strain. But this plant was in a hempy with cheapo shit coco on canna classic floresnutes with no adds. The plant finished up ok and the problem didn't get so bad that it had a large effect on the quality of the finished plant, no leaves totally died or got majorly worse... maybe it did as it was only the second one i flowered and im unfamiliar with the strain.

I have literally not had a problem in flower up until this, not even a single spot on any leaf, nothing has died... if your a major noob like me, give canna coco a shot.

How long has this problem been going on? Noticed near 3 weeks ago
Are you growing in a PVC grow tent? (example: Hydrohut or any other non brand tents)Tent yes, not sure if its PVC though
What system are you running? (DWC? Ebb flow? Aero? Water Farm? Flood Tables? and so on...) Canna coco straight
What STRAIN are you growing? Unknown
What was the establishing technique? (Were the seed or clone?) Clone
What is the age of your plants? Day 47 bloom
How long have they been in there mixture they are in now? around 12 weeks?(coco,soiless etc..)
How tall are the plants? 77cm (stretch has stopped)
What PHASE are the plants in? (seedling, vegetative or flower) are the plants in? Bloom
What Technique are you using? Handwater coco
What substrate/medium are you using?(Hydroton, RockWool etc.) canna coco
What is the Water temperature?
What color are your roots? White? Brown? Are your roots slimy? white
What Nutrient's are you using?(If growing soiless) Full canna coco range
How much of each nutrient are you using with how much water? *Knowing the brand is very helpful* Feeding at around 500ppm
How often are you feeding? (If using soiless) 2X day, 3 if im not lazy
How often are you giving nutrients? (If using soiless) every watering
If flowering, when did you switch over to using Bloom nutrients? canna coco ab so no REAL classic "switch"
What order are you mixing your nutrients? (example: veg nutes 1st, bloom 2nd ect)
What is the TDS/EC/PPM you are using As of discovery of this prob, 500ppm
What is the pH of the "Tank"? feed goes in at 5.5, because run off come out at 6-7.5
How often are you testing your pH/PPM/EC/TDS? every watering
Are you sure your calibration is correct on your equipment? yes
When was your last watering? today
What is your water temps?
When was your last feeding change? (ie. grow-bloom-micro-additional)
How often do you clean your system: example: Flush out water replace with clean water and nutrients? Flushed this plant with a low ppm feed when i discovered this prob
What size bulb are you using? 400w HPS
How old is the bulbs you are using? 48 days
What is the distance to the canopy? 1 1/2 - 2 ft
What is your RH Factor(Relative Humidity)? 40 during day, 60 at night, cant get it any lower at night ATM, BUT air flow is VERY good(update: managed to lower night RH to 60%)
What is the canopy temperature? 20c - 30c
What is the Day/Night Temp? (Include flucutaion range) 18 - 23c
What is the current Air Flow? (cfm etc.) unsure of CFM but have intake/outtake fans and large pedestal fan inside
Tell us about your ventilation, intake exhaust and when its running and not running ? 24/7
Is the fan blowing directly at plants? no
Is your water HARD or SOFT? 100ppm
What water are you using? Reverse Osmosis (RO)? Tap? Bottled? Well water? Distilled? Mineral Water? tap
If using tap water, what is the ppm/EC/TDS of the water right out of the tap? 100ppm
If using RO,Distilled,mineral water, what is the ppm/EC/TDS without any additives?
Are you using water from a water softener?
Has plant been recently pruned, cloned off of or pinched no
Have any pest chemicals been used? If so, What and When? been adding one drop of azamax to feed water for nats for a couple of day about 3weeks ago.
Are plant's infected with pest's? dont know about INFESTED, but there are some gnats here and there.

Clicking on this will open the whole albem there is only one plant affected(one in the close-ups), but there is another pic that shows plants a week behind not displaying this problem:

looks like MG def probably caused by low ph, do you vary your ph between 5.8-6.5 ?

EDIT: pls let me know if you can't see all the pics when you click on the pic

Many thanks for replying.

I now just feed at 5.8, right in the middle of the range(if thats correct), more detail below:

Shit I completely forgot to paste in the OP the PH swing I've been experiencing:

Run off is coming out between 7 -7.5, and that happens whether i feed down at 5.0 or, 6.0, same thing when i was feeding up at 1100ppm(before the flush) before this problem presented itself: 7.0 - 7.5.

I wasn't giving the PH swing too much though because, 1: ive read that as long as your feeding regularly enough/right amount of run off, that it really only matters what the feed PH is and not runoff, if it swings up, it just meant the plant was eating.. 2: The plants looked perfectly healthy and there has been no stoppage of growth. Do i have this completely wrong and should i start feeding up at 6 even with my runoff?

The 2 other plants in the tent(same cuts) being fed same everything, also experiencing this same PH swing but have not shown these leaf symptoms, BUT as their a week behind SAID plant and i flushed and reduced feed ppm as stated in an above post .

Thanks.

i never take any notice of the run off, if you don't vary your PH between 5.8-6.5 (coco ph ranges) you will lock out Calcium and MG.
i grow in coco using canna A/B and twice a week i PH my feed at 6.5 so i dont lock out Calcium and magnesium, i think you have been PH ing to low

Sounds like a top idea, makes allot of sense, 2 birds with one stone, may fix my issue and may improve growth in future coco, much thanks, ill report back how things go

OK so im now watering at 6.0 - 6.2, how long till i see any improvement or change, will the leaves improve in colour or are the permanently discoloured?

The problem is worsening and has now started to show all over the top fans of a second plant that was a week behind, but i was feeding that at above stated low PH.

It almost seems that when these plants reach this stage in flowering this problem occurs, almost like they need either a lot of a certain something or to cut right back on something, as the same cut as these have done this in the past, just not nearly as bad.

Now that its been 6 days since the flush and feed reduction the problem isn't improving, so its a good chance i wasn't burning them after all? That being said, following canna's schedual, i was only giving half dose P/K on the PK week, maybe i should have just gave the proper dose, maybe this is P?K related, but I'm still thinking its PH as that makes the most sense.

Thanks again for any help.

Update

Ive flushed and set PH to 6 but the problem is getting worse and worse, anyone have any idea?

Problem has worsened even further all upper fans now have this yellowing and now rusty blotching, and now happening to a second plant also help please.