I personally no longer use substrate in my tanks. I find the tanks stay cleaner without a place for food and waste to get trapped in. If you really would like to have a substrate in your tank I would recommend using crushed coral. It is a much better than using gravel.
Gravel can get impacted in their digestive system. In the past I used crushed coral for several years without any problems. I used a small size Florida crushed coral (1/8” to 1/4”). Crushed coral can be found at most saltwater fish stores.
Crushed coral is also a great source of calcium and it will wear down their beaks as well. In the wild, adult females can be seen grazing on coral and crushed shells, especially in the spring before they become gravid.
Florida Crushed Coral 1/8" to 1/4"