True Soapfish:
Their skin produces a bitter toxin that is called grammistin. When they are stressed, they release a ton of mucus. If put in a tank, this mucus can suffocate every fish inside, including itself.
Gar Eggs:
While gars are edible, their eggs are not. Their eggs contain a toxin called ichthyotoxin, which can cause severe gastrointestinal distress and sickness.
Greenland Shark:
Their flesh is toxic when fresh due to them not having a urinary system, mking thier muscles get saturated with high amounts of urea and thimethylamine oxide. However, they are techinally edible after a purification process that takes months to do.