Aquarium lice/ Argulus complete guide / Solution

Goldfish lice (Argulus trilineatus) are a member of the Copepod family, so are not actually lice at all but crustaceans.
Fully grown they are 3-4mm round and translucent. This makes them quite hard to see as they fasten themselves flat against the body of their host. The female (shown) is slightly larger than the male.
A heavy infestation can weaken a goldfish from blood loss to the point where it will die.
Signs of infestation are when fish are seen rubbing themselves vigorously against any available object in the pond or aquarium. This behavior often causes injury to scales and fins and is more dangerous to the fish than the lice themselves.