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 piranha. The swarm can't regain Hit Points or gain Temporary Hit Points.
Water Breathing. The swarm can breathe only underwater.
Bites. Melee Attack Roll: +5 (with Advantage if the target doesn't have all its Hit Points), reach 5 ft. Hit: 8 (2d4 + 3) Piercing damage, or 5 (1d4 + 3) Piercing damage if the swarm is Bloodied.