I would ask why you want to? THat's plant substrate; your flowerhorn will probably not let you have too much in the way of plants. Large cichlids like that love to uproot them and shred them up. Also it will lower your pH; flowerhorns, though Frankenstein hybrid fish, are thought to be comprised of CA cichlids, which like harder, higher pH water. So the substrate would be detrimental if anything (though, flowerhorns are tough and adaptable, and this may be fine)
Also, the stuff is quite expensive for a large tank. So unless I got a few bags free, I wouldn't be using it for a FH tank