Mopani typically has a two-tone grain to it and never floats, it's quite dense.
Looks like a cypress root maybe? Not sure.
Is cypress not good for fish?
55 Gallon Planted
3 GBR, 2 Bolivian Rams, 1 Juvenile Super Red BN Plecos, 9 Oto Catfish, 9 Bloodfin Tetras
Rotala Indica, Cardamine, Crypt Spiralis, Pygmy Chain Swords, Dwarf Sag, Lutea Crypt, Wendtti Bronze and Red, and Floating Frogbit