HELP! Phosphorus or Calcium deficency? Advice?

[kilimanjaro]

Hi guys.

First grow here and need some advice.

My first grow has been going well up until this point. I started seeing some dark spots on the leaves and reddish leaf stems. After some digging my conclusion is that the plant has some Phosphorus or Calcium deficiency.

Also, I'm using a crappy PH soil meter that a month ago was reading 6.0 to 6.5 and now I'm seeing 5. Watered only using distilled water and got another batch of it 2 weeks ago.
Regarding the watering part, what I think I may have done is water the plants a bit too early, not letting the first inches of soil dry up.

I grow in basic plant soil that contains peat, coco fibers, clay and limestone + 30% perlite and Biobizz nutrients.
I'll put some pics down below.

Any advice on how to deal with this? What got me a bit worried was the black spots on the buds.

Was thinking of getting some Cal-Mag and add it to the feed along side with lowering the Nitrogen. For measuring the PH, I'll get a pen, the current thingy that I use doesn't look to reliable.
But I also want to hear some other thoughts, as I'm not 100% sure.

[Rwise]

You need to work on the PH, they are going to go into lockout, maybe the meter is near correct and something in the soil is now depleted. When you say "limestone" is it rocks, gravel, powder etc.? Lime should help raise the PH, potash also.
The red/purple in the steams is most likely a reaction to light, or it can be genetic.

[MeEasy]

I'm not sure but it could be related to the distilled water, it lacks the calcium, magnesium, and other minerals that are necessary

[Rwise]

A problem with distilled water is the PH is really 5.8 not 7.0

pH electrodes will NOT give accurate pH values in distilled or deionized water because distilled and deionized water do not have enough ions present for the electrode to function properly. ... It is important to keep mind that water (distilled, deionized, or tap) is NOT “pure” (i.e., pH equal to 7).

Normal distilled water has a pH of less than 7.0 and acidic because it dissolves carbon dioxide from the air. But, the healthiest state for the body is slightly alkaline, something that alkaline water ionizers encourage through water output at the pH 9.5 level.

Lots of info on gooooogle.