I used crushed coral, in a stocking, in my filter. Worked great short term. Not sure how often, if ever, you would need to replace it.
I put in a ball the size between a golf ball and a base ball.
It raised mine from 6.4 to 7.0 in a 36. Then the tap PH went back up and I removed it. Not sure but my tap comes from a lake/river between 2 lakes. Only had the tank since 11/17. They drew the lake down severely in December to make room for winter runoff. I don’t understand why this would affect PH but that is what my tests say.