Slate shouldnt raise ph. But definitely good looking rock. Slate and granite shouldnt affect any water parameters tho. I have slate in 3 out of 8 of my african tanks so good choice! What will raise ph are calcium carbonates(limestone, lace rocks, and texas holey rock which is limestone). What I used to do before I switch all my tanks to aragonite sand is buy a bag of crushed coral put it in a media bag and add it in all filters. Worked pretty well. Glad you do the weekly water change, people skip this crucial step and wonder why their fish get sick and die lol.
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