Some species of fishes partially bury themselves in sandy or muddy and Channa gachua is one of them DONT PANIC! It’s normal. they live in burrows during the dry season so he is just doing what they do.
Yes when I had my ones I had sand in the tanks . They are mouthbrooders and one of the easiest snakehead species to breed. My ones just got on with it not a problem.
My gachua looks pretty funny when he just comes out from burying in the sand. He swims around with sand on the top of his body like he doesnt even notice it.