The skin of Ostracion species contains glandular cells that release a mucous secretion containing ostracitoxin, a haemolytic and surfactant-like compound. It is deployed under acute stress, during handling, capture, or death. In water it lyses gill tissue and red blood cells in nearby fish.
On a reef the secretion is diluted almost immediately, so it functions as a close-range deterrent against a predator that has already taken the fish in its mouth. In a closed aquarium volume, the same discharge reaches lethal concentration throughout the system, which is the origin of the aquarium trade warning against keeping boxfish with valuable stock.
The armour is a separate defence. The carapace is a rigid shell of fused hexagonal plates that leaves only openings for the eyes, mouth, gills, fins and tail, and it cannot flex, so boxfish swim almost entirely by sculling with the pectoral and dorsal fins. That gait is slow but unusually stable, and the geometry of the carapace has been studied in engineering contexts, though the popular claim that the shape is inherently drag-optimal has been challenged by later flow work.