Amphibious. The swarm can breathe air and water.

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 firegill. The swarm can't regain hit points or gain temporary hit points.

Bites. Melee Weapon Attack: +6 to hit, reach 0 ft., one target in the swarm's space. Hit: 7 (2d6) piercing damage plus 10 (4d4) fire damage, or 3 (1d6) piercing damage plus 5 (2d4) fire damage if the swarm has half its hit points or fewer.

Fire Breaths (Recharge 5–6). The swarm exhales fire in a 15-foot cone. Each creature in that area must make a DC 10 Dexterity saving throw, taking 17 (5d6) fire damage on a failed save, or half as much damage on a successful one.