How long after u mix the nutes and start feeding does the change occur? Could be just from the plants drinkuo up the nutes they need and leaving the rest
What was the Ph of the remaining water? If you added Water with a 5.8 ph to a mix that had a low ph it's mix wont stay at 5.8. I would say add ur ph water and test Ph then readjust the whole res.
You are now in flower and it will cause your pH to drop instead of rise like when vegging. After i added my nutes to my reservoir the pH would be perfect. The next day it would drop some so i would just add water (Naturally around 6.5-7.0) and it would bring my reservoir back to normal 5.8-6.0. I never added pH up or down and would bring it up and down using this method. Either nutes to bring it down or water to bring it up.
One thing I hate about hps is its hard to take hood pics with that bright yellow light she is now to big to pull out of my closet lol. So I can't take the awsome pics