First grow NFT + Autoflowers: Is this a potassium (K) deficiency?

Hi all, my first grovel for advice for my first grow:

So I have 4 Afghan Kush Ryders in a little NFT setup and 3 of the four plants are doing fine. One however is showing some serious issues and has pretty much lost 2 leaves to spots. All of these plants displayed red stems and all of those stems split as seedlings, I am continuing regardless.



Data:

1. The plants are all 3 weeks old
2. They are all in the same tank (so same nutrients)
3. They are all Autoflowers and quick ones at that
4. Only 1 plant is showing this issue but she was the first to break ground
5. I am growing with 1 part Ionic Grow @~3/4 recommended dose
6. PH is 6.3-6.5 and fairly steady, the Ionic seems to want it to be 6.5
7. The res water is cool to the touch

Reading this forum it sounds like this is Potassium (Red stems + necrosis) but these plants seem too young to be really needing Bloom? I have read that Autoflower plants actually come out of the ground flowering so this would make sense albeit bizarre. I currently have a res change standing for a bit in a container with a dose of Ionics Bloom @ PH 6.0 based on this theory.

Anyone have advice about feeding schedules for Autoflowers?

Is this just lockout caused by PH?
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If you find out what the red stem is caused from please let me know I have auto with something. I gave it dose of bloom nutes today to see if it was the p two. I'M growing in soil though, not sure if it is I'M need of p or not so I would appreciate a message if you find out. Thanks

Adjust your pH to 5.5-5.9. 5.8 - 5.9 is optimum. Adjust after beginning bloom nutes diluted as they are autos and due to begin flowering this week or next on average. Keep a lot of light on them, air exchanges frequently.

yea i usaly get red stems when my ph is off or i have to many salts built up in my soil causeing lock out. good luck figureing it out thuogh and yea im pretty shure auto flowering strains are always supposed to be on bloom nutes. i have never grow a autoflowering strain just what i have heard

Thanks Hotrod, thanks Sticky! I have just changed the res to Bloom nutes as said above. PH is currently 6.0 (dammit - missed by a whisker) so I'll let them settle for a few hours and then try adding a little PH down. I don't suppose that there is a "best practice" way to add PH down to a res is there? I don't like the idea of adding a pint of quite acidic water to the res while the pump is on but all of the instructions I have seen say "preferably with the pump running". I live in a V hard water area (7.4ph outta the tap - I have no EC, next grow gadget next grow) so I am having to add a lot of PH down (1.5 tspn for ~PH 5.8, it is a little counter intuitive, surely at some point I end up mixing poison?

There is a moral: When the guy in the grow shop says you should buy the best nutes in his shop it is (for once in this crappy world) NOT because he wants you to spend more money in his shop but rather that he wants you to grow good plants. The advice about 1K MH and HPS in your 2'*4' space IS upselling - crap is it hard to keep your cash in your pocket lol.