phosphorus issue?

[JonathanStrange]

Good morning all. I've been struggling to get my plants healthy for a couple weeks now, thought it might be time to post here.
Setup: bubbleponics style top fed dwc. with coco as a medium
Light: 300 W led
Environment: slightly colder than perfect specs, between 60 and 68 degrees, and 70% RH on average. 3x3 tent.
I'vs been trying to remediate the spots that have been developing on my lower leaves. Originally, with some help of some kind folks here, we diagnosed it as a cal/mag issue, and I've added the necessary supplement for the past 2 weeks. While I was checking on them yesterday, I noticed some of the leaves had become very brittle and were breaking off. Going through the symptom checker here has me thinking I may really have a phosphorus deficiency. What could I do to help remedy this situation? Pictures attached.

Thank you all in advance!

Strange.

[JonathanStrange]

As a follow up on their environment, let me describe my typical practices:
The plants are flushed once a week with new water+nutrient solution following Nebula's hydro nutrient schedule, including calimagic
The pH has been maintained in the 5.5-6.5 range for their entire life, checked regularly, if not daily.
I have had the water pump off for the past two weeks, because I've been afraid I was overwatering them, and it was causing a the phosphorus problem.

I hope this extra info makes diagnosing and suggesting solutions a little simpler for you.

Thanks again!

S.

[farmerjim]

I'm 1 month into my first grow ever (also DWC) so certainly not a veteran but looks like a Ca deficiency to me...

Here's a nice pH writeup that shows the pH ranges each nutrients thrive in. If you're pH is leaning more on the <6.0 side and you are not using a CalMag supplement then that's likely your issue (says the newbie)...

What nutrients and supplements are you using?

farmerjim