Water Conditions

I have just recently decided to upgrade my tank to a planted tank and my first step was to replace the substrate with Oil Dri / SMS. Ever since I did this my PH has been below 6.0 and it has me worried.

I got a gh and kh test kit, my gh is about 8 and my kh is under 1

tap water is ph 7.6 gh 10 kh 3

I keep doing water changes but %15 WC only brings my ph up to 6.4 and in two days it's back under 6.0

I'm thinking of taking out all of this substrate and soaking it in a baking soda solution