When using the aquarium kit to test ph, you can't use it for accurately testing run-off. The tinting of the water skews the color-coded results. Clear water only.
Pro Mix is a stable medium and doubtful the ph is crashing already. Which Pro Mix formula are you using? Would raise your water's ph to 6.7ish on watering days, perhaps 6.9 on feeding days. (test before before adding nutrients)

Looks like ph is too low, light is too high, and watering should be deep enough to add moisture to the lower root zones. Heat and fans require more waterings, or more water added per watering. Be sure to get the water to the lower root zones with every watering, but don't keep the roots soggy.

Personally, I'm not a big fan of a 24/0 schedule, and I have had the best results using 18/6.

Find the AN nutrient schedule online, and starting today...stick to it. :thumbsup: