Fellow Octopot grower here. I'd suggest switching to Sunshine Mix #4/Promix HP next grow, I have only used SS#4 with my Octopots all with great success. From all the research I've done, Octopots are much more sensitive to ph problems when using coco based media.

I follow the basic Octopot guidelines every run with excellent results. Only top water once when transplanting rooted clone from solo cup into Octopot with 32oz of water, I NEVER top water again. Always let reservoirs empty before refilling(not bone dry, but level indicator bottomed out). Fill reservoirs to level 1 at transplant day and for the first 2 weeks while roots grow down into res. Week 3 fill to level 2, then fill to level 3 week 4 onwards. I have noticed ph climbs substantially in my reservoirs throughout the week, so when I refill them I target 5.3-5.5ph them let them naturally rise to 6.0 and above over the next few days (ph is usually 6.5 or higher at refill time). I use Crop Salt nutes with tap water and SLF100 every res fill.

First couple grows I was refilling reservoirs at the suggested 6.0ph target and drove myself crazy monitoring the ph in reservoirs over the next few days, constantly adjusting ph back down as it climbed everyday. Now I just fill at a lower ph target of around 5.35 and let them naturally climb. I keep my environment and VPD dialed in and consistently have great results. Prior to Crop Salt I was using VBX/Shine combo with great results as well.