Marble will raise pH. I don't know how much but worth a shot. Also, I found a dosing for baking soda. It's 1 TEAspoon per 5 gallons of water. Add additional TEAspoons as necessary to raise it. Dilute the baking soda in dechlorinated water then add to the tank.
Start out slow so you don't raise the pH to fast and harm your fish.
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