Argulus is a crustacean or branchiurian parasite most commonly encountered in ponds, but they are also found in aquaria. They are easily detected when they strike. They are greenish disc shaped organisms with suckers and small legs. They even have a pair of eye spots on the anterior end. They spend their time darting around in the water away from, and also directly on the fish. They lay their eggs in tubular structures on the glass and ornaments.
They can be very destructive to fish stocks.
They carry Aeromonas and other bacteria on their feeding stiletto and thus infect each fish they bite.
Treatment is by the application of the insect growth regulator, Diflubenzuron.
Another method is more dangerous: Organophosphates like Trichlorfon, masoten, Dylox, Dipterex, etc. I resist the use of these, because losses may result.
Anchors Away is Diflubenzuron. Diflubenzuron is superior to these compounds when fighting Argulus.