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.
Infested fish introduced into the pond or aquarium is an obvious source, but plants gathered from the wild that haven’t been sterilized are often the culprit.
Dipterex (Dylox neguvon) is effective in combating lice.
It is an insecticide that is soluble in water, comes in liquid or powder form and comes in different potencies ranging from 40 to 98%.
Try and find Dipterex with 98% potency as the doses below are based on this potency level. If you can only find lower potency levels, you will have to calculate how much you need based on the doses below.
The treatments for pond and aquarium are different, but the dosage is the same for each. Below are several measurement options:
Be accurate with measuring the dose because above 1.60 ppm Dipterex is acutely toxic to fish. For freshwater invertebrates it is toxic in ppb (parts per billion) so dose on the low side rather than the high.
Treat the pond three times at 7 day intervals.
There should be some form of water circulation to spread the Dipterex evenly throughout the pond.
Dipterex isn’t stable in water and breaks down after a few days. The lice eggs survive the treatment but emerging parasites are killed by the follow-up treatments.
In an aquarium, the lice normally drop off the goldfish within two hours. After this happens, move the fish into a mild salt bath of two teaspoons per gallon of non-iodized salt for a couple of days.
Another treatment option available, especially if the infestation is only one or two lice, is dabbing the louse with a cotton bud that has been dipped in a 50/50 solution of Kerosene and Turpentine. Be careful not to get any of the solution into the gills, mouth or eyes of the fish.
The louse may still need to be removed with tweezers.
Sterilize the aquarium completely (including plants) to kill any adults or nauplii.
A safer more convenient treatment is to use something like Ecological Labs LAWP16 Microbe Lift Lice and Anchor Worm treatment.
As stated on the bottle, this medication will also treat anchor worm which is a convenient bonus but not surprising as both are crustaceans.
Follow the manufacturer's instructions and monitor your goldfish carefully for signs of toxic reaction such as lethargy, gasping at the surface or turning on its side.