Interesting, my FH never interacted with the substrate (gravel) when I had it in my tanks. This was several fish over the years, not just one. They all started with gravel. Removing it later in the game seemed to make no difference whatsoever to the fish. Perhaps they were all born & raised in bare tanks in Thailand, not sure? My current midas (almost 8 yrs old) also has never payed any attention to sand or gravel. These fish all flew solo, so perhaps with a female around they would have acted differently with regards to cleaning spawning spots etc, or maybe they are just all oddballs, but by themselves, it seemed to make no difference whatsoever to the fish.