You do not want crushed sand or gravel Knox. It ends up with sharp edges which is exactly what people buy sand to avoid. When working with catfish, the whole point to having sand is to be gently with the fish's barbels.
The earth eating cichlids, geophagus, are another kind of fish that appreciates a fine gravel or sand.
The earth eating cichlids, geophagus, are another kind of fish that appreciates a fine gravel or sand.