I bought every piece of HITH lore I could find and ran with it. No carbon in the filtration, no stray electrical current, no bad food (like Hikari Cichlid Gold Corn Flakes, but you feed him other stuff, too, so...), stable pH, nitrAtes below 20ppm, low stress, lots of insects in the diet, etc, and he still got a hole one day!!!
What worked was a single dose of metronidazole followed by adding a vitamin and mineral pyramid to the filter box. And it hasn't come back since.