Can hookworms possibly infect fish? Is it visibly noticable? I understand it's an internal parasite and you would likely need a microscope to view the parasite? Could it be mistaken for Anchor worms? A customer at my work suggested her fish got hook worms from a fish we sold her.. It killed her whole tank, except 1 cory cat. She dosn't want to clean her tank because the Cory cat enjoys the dirty tank and she dosn't want to 'overclean' and harm her delicate balance. With that being said, could these 'hook worms' just be planaria from poor husbandry practices, and her fish died simply from poor water quality? She purchased livebearers, and when introduced to a tank with poor water quality I find them to be very susceptible to fin/tail rot and other secondary infections.