Hello,
i also keeping my bichirs with black substrate. Looks cool and is helping to give the bichir's colouration to its best ...
But: Black gravel ISN'T black gravel! Mine is a more "synthetic" Product, animal-friendly due to missing sharp edges!
Unfortunately it is even quite common to offer this OBSIDIAN sand or gravel in pet shops. Good way to kill bottom dwellers!
Obsidian is a vulcanic product, often sandy on those "dream coasts" - i think there are also any islands in the Pacific with beaches full of them. In the more European hemisphere The island Teneriffa (Baleares) is one of the most popular examples of this type of beach.
But back to the unfortunate circumcisions: This black gravel looks cool on the beach - and it looks cool in the tank - and kills everyone who eats a bit from the gravel. Razor-sharp (Obsidian do have a very sharp way to break, much more "efficient" than normal glass!) it will cut its way through the unlucky gourmet.
So, please, look WHAT kind of gravel you have!!!
Greetings
Uwe