this is actually my senegal
I used to have him on tan/brown gravel and he seemed just as dark to me as he does on black sand now. However, the yellow speckling along his midline became much more pronounced. Honestly I never noticed it until I got the black sand.
As far as good goes, he generally eats live feeders, occasionally bloodworms, mysis shrimp and he will eat the pellets I give the rest of the fish, since they are in a community tank. This is tetra color tropical granules.
I just started working at the pet shop where I got this particular senegal and set up a display tank (they know me so well there that they gave me my own display tank on the second day) in hopes of drawing more attention to polypterus. I used black sand and added two senegal bichir, one that was a lighter color, and another that was darker. I'd like to see how they look when they are full size and see if they stay darker and lighter.
I picked my original senegal at random about 3 years ago, mostly just chose the healthiest looking one and I am very happy with him. He is still my favorite out of all my polypterus (i have 5). He's just so beautiful, I'm glad others like him as well!