API General Cure contains Praziquantel and Metronidazole.
Metronidazole does nothing to intestinal worms and is used to treat internal protozoan infections in people and fish.
Praziquantel treats tapeworm in all animals, birds and fish. Praziquantel does not treat thread/ round worms in any animal or fish, it only treats tapeworm.
If you need to treat round/ thread worms, use Levamisole or Flubendazole. Levamisole treats most common thread/ round worms in fish. Flubendazole treats thread and tape worms in fish.
Flubendazole is available in the US and is the better treatment because it does all intestinal worms that affect fish.
Deworming medications are ingested within a few minutes of being added to the tank and fish are often seen pooping out the dead worms a few hours later. All worms should have been expelled within 24 hours of treatment.
It's a good idea to do a complete gravel clean and huge water change 24 hours after treatment. This removes the worms that have been expelled.
You should also treat all the tanks at the same time to prevent cross contamination. And treat them once a week for 4 weeks to kill any worms that hatch from eggs in the fish.