I've never kept Apogon melas, but I have kept the 2 native species of black cardinals, Astrapogon puncticulatus, the atlantic black cardinal, and A. stellatus, the conchfish. They are both black and have big pelvic fins, care is the same as any of the other nocternal cardinals. they should be kept singly though they are not that agressive openly they will stress eachother out unless you have a big tank and lots of hiding places. Both of these fish are found living inside a host, the black cardinal in the tubes of vase sponges and the conchfish in the mantle of the living queen conch.
They are not hard to care for, accepting any food including all frozen and flake, but both of these sp and A. melas as well are nocternal and tend to spend the daylight hours hanging out in a dark area of the tank, I set up a dark corner that I could look into and the fish hung out there. When the lights were off, except for the moonlight led's the fish swam about and foraged on the live rock for small inverts, when I started losing small shrimp like my thor shrimp I thought it was him as they have big mouths so I gave it away.
