RO water tested at 4.0

I'll be honest. I never checked my PH during this entire grow cycle. I vegged for 2 months in a straight mixture of happy frog and ocean forest and only supplemented calmag with every feeding. My plants were going nuts during veg and didn't show a single deficiency, so I figured my PH was perfect. Well, today makes 5 weeks into flower and out of nowhere my lower fan leaves are yellowing and burning to a crisp. I'm still learning to identify plant issues and right away I narrowed it down to potassium deficiency or PH issues. My buddy works for the water treatment center i get my RO water from so I had him test the PH for me. He said that most RO water come in right around 5-5.5, but we were both shocked when he tested and it came back at 4.0. I'm going to buy some test strips and test my well water. I haven't wanted to use water from my well because we live directly across from a farm and they are constantly spraying their crops with who the fuck knows what and I'm worried my ground water is leaching up all the bullshit they're spraying. Should i keep using RO water but boost the ph with baking soda or just use my well water? Should i flush?!?!? I'm so confused and worried. My babies only have a few weeks left before they get the chop.

Update. Went and bought some ph drops. RO water tested right around 6-6.2. My buddy's drops must be old. After diagnosing some more, it appears to be a phosphorus deficiency. only affecting lower leaves that have bright red stems. Any suggestions how to quickly boost phosphorus organically?

Bone meal or rock phosphate.

Also sea bird guano 0-12-0

i gave em a tea this morning with bloom dry amendments. I think one of the main reasons i'm running into this issue is because my plants are defiantly outgrown in their 5 gallon pots and my humidity was way to low last week. sitting around 45%. I was literally forced to water every day and i think that washed out a lot of nutes.

Mammoth P