My biggest (4 weeks into flowering )plant's fan leaves started turning obvious yellow a few days ago so I was planning on using some florashield with the next feeding time to see if it would help.

I'm wondering if mixing that in with my bloom nute (+50 drops of ph down in the water to get it down to a 6.9 for soil) cause any harm???

anyone have an answer?

Adding a little bit of FloraShield shouldn't cause you any problems but I don't think FloraShield will solve your yellowing problem either.

You need 50 drops of pH down to bring your pH down to 6.9?!? that seems like an awful lot to me. i'm assuming you're using tap water. you may want to pour your water into a reservoir and let it sit for 24 hrs. add your nutrients and mix everything up really well. wait for a day and check the pH and adjust accordingly. I am recommending that cause the nutes will effect pH over time.

you should also check out this page:
Marijuana Plant Abuse

to see if you can determine the root cause of your problem.

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thank you very much!

yes, i am using tap water but ill be switching water sources soon.