Hello, Flowerhorns like to rearrange their substrata, so rocks and driftwood and ornaments on sand or bare bottom. (sounds like a nude beach) if you get gravel, get rounded gravel pebbles. River Gems sells some nice looking agate substrate. The bags are sold in 2 and 5lb bags. The bags have a mix of small agates, that go from beige to milky white to clear. Forget the exact price. Reasonable though. You can get rounded gravel in any color. I have white sand mixed with sky blue round gravel. My cichlid is constantly rearranging. Looks nice with occasionally changing vistas. I have the River Gem agates in the other tank.
Since flowerhorns and some other fish use their mouth to move the substrata, best to use something round so as not to scrape or cut the inside of their mouth.
If you have a light colored fish you might go for a darker color substrata to make your fish stand out. Live plants are not adviced as they will uproot and/or eat them. I have underwater gardens in several of my tanks. Except in the cichlid tanks. In one I have nice looking fake plants. I do throw in an occasional sprig of water wisteria or narrow leaf Ludvigia for them to munch on.