Mad Doctor Fish

Mad Doctor Fish (マッドドクターフィッシュ, Maddo Dokutā Fisshu) were Mysterious Beings and part of the Monster Association. They lived in a sort of symbiotic relationship with Evil Natural Water, but were killed via oil poisoning.

Appearance
The Mad Doctor Fish's appearances are based on red garras (also known as doctor fish), but they have larger jaws and sharper teeth that bear a resemblance to carnivorous piranhas. They also appear to possess up to six eyes, though the number varies per fish.

Monster Association Arc
As a last resort, Master Joe opens the tank holding Evil Natural Water, releasing it as well as the fishes to attack Atomic Samurai's disciples and Tongara. The fish devour Master Joe, and Evil Natural Water uses them to attack the humans as it rampages down the hallway. Once Tongara is lethally wounded and forced to stay behind, his body and battle suit are torn to shreds by the Mad Doctor Fish.

More Mad Doctor Fish appear when Evil Natural Water attacks Child Emperor, again launching them at its target. They chew up some of Child Emperor's backpack's arms and weapons, but he manages to dispose of most of them. Once Child Emperor freezes Evil Natural Water, it absorbs the oil from Sludge Jellyfish's body to reduce its freezing point; this poisons and kills all the Mad Doctor Fish in the process, and their corpses are expelled.

Abilities and Powers
Because their collective threat level is Tiger, it can be inferred that the Mad Doctor Fish are dangerous, but not extremely powerful. They act as an effective means for Evil Natural Water to conduct a variety of long-ranged attacks.

Physical Abilities
Powerful Bite: Mad Doctor Fish have a strong bite that can easily pierce the flesh of other mysterious beings. They are able to completely devour weaker and wounded opponents in seconds, and even tear apart Tongara's highly durable armor. Mad Doctor Fish could also chew through Child Emperor's metal weapons to break them.

Trivia

 * The Mad Doctor Fish are the only monsters in the series so far to live in symbiosis with another.
 * As of the revision of Chapter 114 into Chapter 110, the Mad Doctor Fish are no longer explicitly identified by name or disaster level.

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