QUICK DIAGNOSIS? Mg/K Deficiency?

Can anyone confirm that this is a Mg deficiency? Maybe Potassium(K)?

The plant is 7 weeks old from germination. Ive given it Fox Farm Grow Big and Big Bloom (in small doses- Never went over 200ppm) twice in the last 3 weeks.
I checked the run-off of the water and the pH is 6.3 and at 237ppm. The water ive given it has been around 6.5 to 6.8, depending on how much my meter has drifted. It just started yellowing about a 8-9 days ago. In addition to the yellow leaves they have also died with copper color spots right before they drop - see pics. The upper part of the stem started getting purple lines. Overall, its not growing at full speed as it should. Thanks in advance for advice!

Also, this morning I mixed 3/4 tsp. of cal-mag plusin a gallon and fed the plants.

Did it work?

Get your pH to ~6.7 when you water, add much more fertilizers. I don't like the ppm counting method as they have too many multiplying factors. Check whats yours from the EC and give them around 1.5 mS EC (1500ppm or 750 ppm, or ~1100 ppm, depending on your meter) every second watering. It seems like they lack of K (not enough blooming feed) and lack of Iron (pH issues usually)

For all those that helped I figured it out. Maybe other who are going through similar situation can learn from this. I was using tap water to water everything and found out that the issues were coming from high calcium levels. The calcium was basically locking out my magnesium, phosphorus, nitrogen. I did what other people said, like "your plants look underfed, feed them!" and I fed them made it worse. The I decided to flush them, still not knowing of the high calcium. The plants looked a little greener a couple days after flushing. However, when the pots started to dry up the plants got even worst. So I decided to flush them again! Mind you, Im flushing with my tap water that has been properly pH'd. Decided to look at my drain couple days ago and boom, caked on calcium deposits around the bathtub drain. So now Im waiting for the second flush to completely dry and I will try again with distilled water... Im posting this because if I had read this post while i was having problems I could have maybe made a more accurate diagnosis, or at least added it to what I had already read. Good Luck guys and remember not everyones tap water is the same, obviously. Some has more minerals and nutrients than others. In my case, lots of calcium.