Solid Camouflage. The reef stonefish appears so similar to a lump of rock when not moving that it can almost effortlessly hide against the bare earth. In a natural environment consisting of stone, mud, sand and rock, it has advantage on stealth checks even if there is no solid cover to hide behind.

Water Breathing. The reef stonefish can breathe only underwater, but can survive out of water for up to 24 hours.

Most Naturally Venomous Fish In The World. Unarmed melee attack rolls and grapple attempts against the reef stonefish risk the poisoned condition and eventually, death.

If either of these actions are made against the reef stonefish the attacker must make a (DC 16) Dexterity saving throw after the attack or suffer (3d10) poison damage and gain the poisoned condition which the attacker can roll a saving throw against only once every 10 minutes with a (DC 18) Constitution saving throw. They will also gain one point of exhaustion every 10 minutes until death unless the attacker succeeds a saving throw against the poison condition. The accumulation of exhaustion points from the reef stonefish venom stops when the poison condition ends. 

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 1 piercing damage.

Ambush. If the reef stonefish surprises a creature and hits it with a bite attack, the target takes an extra 2 (1d4) damage from the attack.

Squirt Water. Ranged Weapon Attack: +5 to hit, reach 5/10 ft., one target. Hit: 0 damage.  This fish can spit a tine stream of water at people. It doesn't do any damage, but it's a fairly unusual ability for a natural fish to have.