Water Breathing. Slaughterfish can breathe only underwater. Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny slaughterfish. The swarm can't regain hit points or gain temporary hit points.
Bites (swarm has more than half HP). Melee Weapon Attack: +2 to hit, reach 0 ft., one target in the swarm's space. Hit: (2d6) piercing damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or contract a disease. Bites (swarm has half HP or less). Melee Weapon Attack: +2 to hit, reach 0 ft., one target in the swarm's space. Hit: (1d6) piercing damage. If the target is a creature, it must succeed on a DC 10 Constitution saving throw or contract a disease.