Purple Underside Leaves And Stem On Tomato Seedling.

I am almost 2 weeks into growing my cherry tomato seedlings in hydro. In the pictures they're different sizes because some of them were planted between 5-10 days apart from one another. However, the oldest one which has the most leaves, seems to have its leaf tips curled upwards and the underside now is completely purple! I have been giving them seedling/cutting part A+B nutrients by DNF. Ive been given them only 1/4 strength, and the NPK on the bottle shows 2-0-2 on A and 0-1-3 on B. I also adjust the pH downwards everyday using apple cidar vinegar.

I did some research and the only problem i found associated to this was a phosphorus deficiency. I've posted some pics, i hope someone can give me an answer or solution. I was thinking just add a little bit more of solution B to increase phosphorus, but that will also increase potassium as well, so before i start playing with nutes I thought id get some advice from you guys first on what to do. and yes this is my first grow in hydro lol a little more about the setup...

5 - 16watt daylight CFL's (will be adding alot more and higher wattage when these guys get a bit bigger, have y apapters coming in atm)

ebb&flow system, 2.5 gallon reservoir.

if you need more info, let me know.

Yep yep, its perfectly OK, it is turning into wood...

My tomato plant has been turning purple all week... PLanted upside down under a hanging bucket, the stem is now as thick as my little finger. Plants planted in various soil spots are maybe as thick as two matchstick, despite being exactly as old as that one.

phosphorus deficiency most likely

when should nutes be added to cherry tomato seedlings? I don't want to jump the gun and end up with nute burn.

We seem to talk about two things...

that sucker is starving. tomatoes need calcium, so you needed to be basifying instead of acidifying. at certain pH's some nutes aren't readily available to the plant. start feeding with your nutes at full strength. then switch to a more potent formulation ie. 13-13-13 at 300ppm to grow on. supplement calcium to prevent blossom end rot. and you should be fine.