Leaves browning in multiple spots Lower leaves yellowing but have green veins.

Certainly has all the earmarks of septoria and is caused from over watering / too frequent watering's / damp cool climate / not enough air movement around the plant ...You need to clear this up ASAP or other nasty things will follow. .... The fact that your other plants are not suffering just means the fungus hasn't reached them - yet. I would offer up some extra 02 with a mix of 1-2 mils of 29% HYDR-OX ( food grade peroxide ) with 1 gallon of water ...add a bit into the already wet soil ..it will offer up extra 02 to the roots and media until the soil is dry enough to water & feed once again ....Raise temps to around 75-78F & get fans ..extra if needed to get your ventilation moving ...it will help to dry the soil out. Another thing I would do is to clean the grow area ..the cleaner you keep it, the less chance of pathogens ......hope this help's you out and you clear up your problem.... Good luck!

this chart may help you identifying the issue(s): https://loudclouds.co/wp-content/uploads/2016/07/marijuana-deficiency-chart-jorge-cervantes.jpg if you were low on nutrient feeding would say that's prolly both N and K deficiency (may be calcium and magnesium also) 6,5 is perfect pH for soil, range is 6,2-7,0 happy growing

Additional Notes: - Until the problem started occurring, I wasn't giving the plants many liquid nutrients at all. Only started once I recognized the issue. - Only this one plant has shown any issue of this nature, they all receive the same water and treatment. - After getting the okay pH and TDS readings, I started increasing the liquid nutrients a little since I started to assume it was a deficiency. - Unsure if a 6.5 pH reading is too low for soil. Despite that, the other plant isn't showing many issues; if any at all.