Having trouble diagnosing (leaves slowly dieing)

Hi all first time grower and ran into a little bit of trouble, i spent about 6 months or so reading up but thought well only real way to learn is to get down to it and actually grow some.
well problem is my lower largest fan leaves on 1 plant started getting small yellowish dots around the edge of the leaf between the veins about 1 week before flowering started, (not tip like nute burn but near the edge), these slowly started spreading over that leaf in the next few days and the now older yellow spots got larger and started turning brown, this is also when the leaf edges and very tip started to curl upward, i looked online and found this could be common case of mg of cal def, i got some cal/mag, applied to 55ppm raised it to 150 (which ive read is around average for soft water), added quarter strength nutes (as recommended with auto's and they had no problems so far) but the problem has persisted and spread further and just got worst, PH is kept at a constant 5.7-6 never below or above that, res temps have always been 18-22 degree's (only reaches 22 near when it needs a new ice bottle to cool the water which is checked regularly), res is changed every 4-6 days, humidity throughout grow has been 30-35%, here is some grow setup info and pictures.

Running a 250w hps light (dual spec), got intake and extraction 100mm rvk fans controlled by rhino fan controller, temps are always around 22-23 degrees when lights on and drop to 20-21 when lights are off.

Grow setup is a NFT gt205 grow tank, (pump on 24/7, has air stone in res) 2 plants both same strain, sweet auto skunk, there both into first week of flower now (about 4 weeks old).

Nutrient im using are vitalink max part a and b (grow and bloom), vitalink bioplus, i also use tap water which comes out at 55ppm, i normally leave the water to stand over-night.

Well seedlings have been completely healthy and all the way through veg was healthy (got some minor brown speckling on top of one plant, moved light further back all new growth was fine so assume light stress there), started veg with 250ppm, raised to 300 two weeks later once i started noticing the ppm dropping faster, kept the same till i noticed them flowering, changed over to flowering nutes and went with the 350ppm, 1 plant seems to have taken perfectly to all of this, except for the strange problem i have with the other one i just dont understand?

Things ive tried: flushing completely with fresh water incase of over-feed, plants just started drooping after a couple of hours looking hungry, started nutes back 100ppm at a time untill normal levels and nothing changed except plant wasnt drooping, tried flushing pure water with just cal/mag, tried using cal/mag as supplement with nutrients, i have tried giving stronger feed as i thought it may be them not getting enough of something in the nutrients, went up to 450ppm and i started noticing white tip burn so i assumed nute burn and flushed, went back to normal nutes.

Also the pictures are 2 days old, the leaves you see affected are now completely bright yellow on the edges and brown in the middles, extremely dry and cracking/falling off, i will try to get pictures of that tomorrow for a bit more of an update.

If any more information is needed just ask and i will do my best to provide it, i have posted everything i can think of here that ive done or has happened but i may have missed something silly, help would be extremely appreciated!

Check pics of P def. But they should have plenty as most nutes packs have it.

Yeah ive had an in-depth look for several days as it got worse to try and diagnose it myself via the pictures, but nothing seems to fit exactly, as i said ive tried going up with nutes but it seems to result in tip burn, the leaves now have gone extremely yellow and brown (the worst ones in the pictures provided), completely dried and dead, the closest i could find that matched my problem was MG missing, but i'm using cal/mag now to bring my 55ppm tap water to 150ppm (or as close to) before nutes, then raising up to 350 with bloom nutes, anything more seems to burn the already sick one.

It almost reminds me of a mold. Do you have a lot of air movement? This is tough because you seem to have covered all the bases. You shouldn't vary the nutes to much as that can shock your plants. I would go back to your original nute levels and just keep the cal/mag in the mix. Sorry I'm not much help.

Well i custom built a small walk in cupboard for the gt205 tank, the size of the room is the size of the tank pretty much (hence growing auto's not alot of room), its about 1.6m high, run both fans around 50-60%, i can feel air moving around if i put my hand in there, wasn't sure if it was worth getting a small desk fan to to blow mid room, the tank sits on a small 5cm high table i built which keeps the cold air from the in-take going under the tank to help keep the water cool and then it blows against the wall (35-40cm away) up-wards, i expected this to be enough as its a very small space?

Also with only 1 plant being affected this way seems very strange, if it was any sort of mould or fungus i would of expected it to spread by now (about 1 week since this started), thank you for the input so far guys, didn't know about the varying nute solutions could be bad so ill make sure i keep that in mind, tbh im not overly fussed if it dies or something happens, first time so its just a learning experience, would just be nice to know the cause to prevent any future mishaps.

Tomorrow i will try to take some pictures of the actual grow cabinet, but i designed it around a tent, intake bottom and extract top etc. its just a wooden version of a tent (to give some idea) will also get pictures of the plant's progress and how (bad) they are now.

Thank you for the input it is extremely appreciated!!

They look burnt at the bottom like you have overfed them, the new growth at the top looks ok,
pull the burnt leaves off at the bottom they are of little use now,
i would have those small plants on .8-1.0 EC some are more sensitive than others
keep an eye on all the new growth , i think as they are small, time is the best thing at this stage
do not give them any more food or additives, ph is fine upto 6.5 , just wait it out for a while they should be growing fast now
aslong as the new growth is healthy , just let them grow it out , see how they are in a week before you add anything
at this stage it is very unlikely they are deficient , they can grow bigger than that on straight water, i have forgot to feed them before and found this out lol
you do not need to worry about an airstone with NFT

i am using a similar system to you, also using the same vitalink max food

Well, i was a little concerned about ph problems (silly since ive been so vigilant) but everywhere else i read with similiar symptoms everyone got a cure via fixing ph, i ordered a pinpoint ph monitor, turns out the ph pen i was using was almost .7 out, so instead of my plants having water that i thought was 5.8 it was more like 5.2, god knows how they survived this long because ive had the ph pen since the start, hopefully all is corrected now and things should be ok! i chopped the dead leaves today, the plant still seems to be growing (all but slowly) and bud sites are popping up everywhere, fingers crossed it carrys on fine.

Thank you for the input!! Is extremely appreciated.