I am setting up a 75gal. aquarium for a plants. I used Seachem Flourite for the substrate. Yesterday I filled the tank with tap water with a KH of 5 and PH of 8.5. I let the filter run for 24 hours and retested the water. It was KH of 3 and PH of 7.4. What would cause this? I then retested the tap water and got KH of 5 and PH of 8.5.