Crushed coral is a great tool for raising your pH. We personally use it at 1 pound of crushed coral per 10 gallons of water when mixed in with our gravel. Crushed coral dissolves over time continually. The lower your pH is, the faster it dissolves. As the pH rises, it slows down how fast the crushed coral dissolves. We find that Crushed coral may need to be replaced every 1-2 years depending on how low of a pH the tap water is.
Crushed coral adds a bit of hardness to the water but not much. It does add a lot of kH which buffers the water from pH changing.
When using it in your filter, crushed coral will dissolve faster. This means you'll usually use smaller amounts but replace it more frequently from to 1-3 months being most common.