Quoting Wikipedia on antibiotics:
"Most anti-bacterial antibiotics do not have activity against viruses[24], fungi, or other microbes. Anti-bacterial antibiotics can be categorized based on their target specificity: "narrow-spectrum" antibiotics target particular types of bacteria, such as Gram-negative or Gram-positive bacteria, while broad-spectrum antibiotics affect a wide range of bacteria."
"Benzoylureas are chemical compounds best known for their use as insecticides. One of the more commonly used benzoylurea pesticides is diflubenzuron. Others include chlorfluazuron, flufenoxuron, hexaflumuron, and triflumuron. Lufenuron is the active compound in flea control medication for pet dogs and cats. Benzoylureas are insect growth regulators which act by inhibiting synthesis of chitin in the body of an insect after the insect consumes it."
As far as the antibiotic metronidazole, i guess its hard to tell if its actually kill the parasite in ur waters because we dont know if the antibiotic will be effective towards it but maybe with a bit of research for what u have and what this antibiotic kills, ull find out