pH swings down really low on Daily Basis

First time grower. There was quite a few mistakes during this grow but I've accounted for most of those and can't figure out this one.

I'm running a 25 gallon recirculating DWC that drops down to around 4.5 pH during and after my light cycle. I pH Up back to around 6-6.2 but by the next day it's drastically dropped again. My nutrients are right around 800. I read somewhere that raising them a little higher might fix the pH drop but going from 600 to 800 had no real difference.

If anyone has advice I would really appreciate it. I've tried searching for issues related to this and didn't come up with anything applicable.
 
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