We generally like to run all of our tanks at a pH of 7.0 to 7.8. If you don't have this range it doesn't mean it can't work for your fish. It's just a great range to be in if your tap water is already there. We recommend not modifying it if you're already in this range.
If you are below this range, we use 1 pound of crushed coral sprinkled into the gravel for each 10 gallons of water to raise the pH slowly over time. This needs to be replaced from time to time, typically 6 to 18 months depending on how acidic the starting water is. If you wish to add crushed coral to your filter, know that it'll raise the pH faster, but also run it's course faster.
If your pH is above 7.8 and you need to lower it, unfortunately that is a much harder task. We recommend using an R/O filter if you absolutely must lower it. An alternative would be to use a planted tank and stop changing water. This will lead to acidification of the water over time. When the water gets to the level you'd like, you can then do very small water changes periodically to maintain that level and safe parameters for your fish.