Identifying a leaf problem

I’m a first time grower and I’m seeing a problem with one of my plants. Early in the flowering one of my three plants, otherwise very healthy looking, started to show dark veins and have brittle leaves. The plants are different strains. All three plants have the same soil and nutrient schedule but only one has the issue. I don’t see any insects involved.

Here are a three pics showing early, mid and late stage of the issue. It’s hard to see but the leaf above the bud is the early stage…

Any suggestions on how to combat this will be appreciated.

Welcome to the forum! You may wish to consult the deficiency charts available online?
We need you to describe your grow entirely
In the top of your screen there is a magnifying glass, you can type in ( support ticket ) that is the format for the questions we need to understand your problen.
Anytime you want to ask a particular person a question or simply talk to them, type ( @wkm ) and that will bring them to the thread.

Thank you for the advise on the forum format. The whole process has been a lot of fun in light of Covid. I’ve looked online at all of the pics of nutrients and diseases and it seems to be closest to Calcium deficiency.
Strain; Type, Bag Seed, Or Na:
Tiger Chameleon (Clone). Planted in early July.

Soil In Pots, Hydroponic, Or Coco?:
Planted in fresh commercial potting soil (three gallon pot) in early July.

System Type?:

Ph Of Runoff Or Solution In Reservoir?:

What Is Strength Of Nutrient Mix? Ec, Or Tds:

Indoor Or Outdoor:
Outdoor - Direct sun currently 6hr/day.

Light System, Size?:

Temps; Day, Night: Days:
88-98F Nights 68-70F

Humidity; Day, Night:
Day: 30-40%, Night 55%

Ventilation System; Yes, No, Size:

Ac, Humidifier, De-humidifier,

Co2; Yes, No

Add Anything Else You Feel Would Help Us Give You A Most Informed Answer. Feel Free To Elaborate:
During the growth phase, I used fertilizers on hand. Miracle Grow general purpose. In the flowering stage, Miracle Grow Bloom Booster. One of my plants is two weeks ahead of the problem plant in flowering and looks completely healthy. I suspect calcium deficiency. Three days ago, after I realized there was a problem, I’ve started adding CAL-MAG to the watering schedule per the instructions on the container.

pH? Of the soil or medium (root zone/reservoir/run-off) and of the water and/or nutrient mix that is fed to the plant?
The best I can tell the pH is between 6.2 and 6.5 measured with a soil test kit.

Type and strength of nutrients used? NPK? EC/TDS/PPM levels? Of what you feed it as well as what is in the water by itself and in the soil/medium (run-off), or reservoir.
I have not measured these levels.