Anchor worm, fish lice and leeches on a goldfish

Most goldfish parasites are too small to see without a microscope, which is what makes them hard. A few are not, and those feel as if they should be the easy ones: there is the parasite, remove it. They are not the easy ones, because what you can see is the last stage of a life that mostly happens in the water.

Three things can be seen clinging to a goldfish. Anchor worm and fish lice are crustaceans, relatives of crabs and shrimp rather than of worms, and leeches are the one true worm among them. All three arrive from outside, on a new fish, a plant, a bag of live food or a bucket of pond water. None of them appears on its own in a closed tank.

First: what is it, and does it move?

Watch the fish for a minute or two in good light. Movement settles most of it.

What you see	Moves?	Most likely	What it means
A short white or greenish thread in a red spot, sometimes with two sacs at the end	No	Anchor worm	Treat the fish and the water
A flat, see-through disc a few millimetres across	Glides about	Fish louse	Treat the fish and the water
A worm that stretches, contracts and loops	Yes	Leech	Remove it; check for more
Small black or yellowish dots under the skin, pond fish	No	Black spot, a fluke larva	Usually leave it
White dots on gill covers and front fin rays, spring	No	Breeding tubercles	Nothing: a mature male
A long string from the vent	Trails	Faeces	Nothing, unless it is white and stringy

The last two rows are common false alarms. A male goldfish in breeding condition grows small white bumps on the gill covers and the leading rays of the pectoral fins every spring, and the breeding page shows how to tell them from a parasite. A trailing string from the vent is faeces, and the constipation page covers when it means something; red threads that withdraw into the vent, or tissue pushed out of it, are on the vent page.

Anchor worm

Anchor worm is a parasitic copepod. Its young stages cling to the gills and skin while they mature, and a heavy load of them can kill a small fish with no thread in sight; only the adult female embeds herself. She drives her head deep into the fish’s muscle, where it branches into the anchor that gives the parasite its name, and the rest of her body stands out of the skin as a short thread that can reach about 2.5 cm. When she is carrying eggs, two small sacs hang from the free end.

Around the base the skin is red and inflamed, and scales are often lifted or missing. The fish frequently rubs the spot against the decor. Left alone, each attachment is a small open wound, and the crater it leaves behind is a common starting point for an ulcer.

Removal is a skill, and the thread is the wrong place to hold. Pulling on the visible body usually breaks it away and leaves the head in the muscle, which keeps the wound open and often infects it. Done properly, removal uses fine tweezers gripping right at the point of attachment, with the fish held gently and briefly, and the wound is treated afterwards. It is standard practice among people who have done it. If you have not, an aquatic vet can do it, or show you once, and that is a better first time than learning on a fish.

Fish lice

A fish louse is a flat, translucent disc a few millimetres across, the colour of the fish beneath it, which is why it is so often missed. It holds on with two suckers, moves about the body, and can let go and swim to another fish. It feeds by piercing the skin, and each feeding site is a small red mark that can become infected.

A fish carrying lice often flashes, rubs, darts or jumps, and in a pond the fish may gather at the inflow or behave as if something is chasing them. The flashing page covers the behaviour, and the point worth taking from it is that flashing usually starts before the lice are noticed.

Lice are easier to dislodge than anchor worm, but lifting them off the fish does not solve anything on its own. Adult females leave the fish to lay their eggs in rows on hard surfaces, on the glass, stones, pipework and, in a pond, the walls and the undersides of lily pads, and those eggs take from about two weeks in warm water to two months in cold to hatch.

Why removing them is never enough

The treatments that work are specific to crustacean parasites, and the things people reach for first do not do it: ordinary doses of aquarium salt will neither clear lice nor kill embedded anchor worms, and an ich treatment targets a different organism entirely. Sustained high salinity can stop anchor-worm larvae developing, but that is something to run with a vet, not a substitute for a crustacean treatment. What is available varies by country — some products are sold for ponds, others only through a vet — and some also affect other invertebrates, which matters in a pond that is home to more than fish.

That is the argument for a vet’s advice rather than a shelf. The medication guide covers why working through products one after another costs a fish more than it saves, and the rules that apply to any treatment: dose for the real water volume, take activated carbon out of the filter, add aeration, and finish the course.

In a tank, a treatment can be run in a hospital tank for a single fish, but the main tank still holds eggs and young, so it needs treating too. In a pond, the pond is the treatment area, and the scale of it is one reason these parasites are harder to clear outdoors.

Leeches

Fish leeches live in ponds, and reach aquariums mostly on plants and in live food. They attach by a sucker at each end, feed on blood, and drop off when they are full. One or two on a healthy fish leave a small wound and are a minor problem, and the hitchhikers page puts them among the uninvited arrivals that are usually harmless. Many at once is different: the wounds add up, small fish lose more blood for their size, and some leeches carry blood parasites between fish.

Remove any leech you find, check the rest of the fish and the plants, and in an indoor tank trace where it came from.

Black spot

Small black or yellowish dots slightly raised under the skin of a pond fish are usually black spot, the resting larva of a fluke. The life cycle needs a snail and a fish-eating bird, so it cannot complete in an aquarium and does not pass between fish in a tank. It is mostly cosmetic, and the lumps and growths page covers it with the other things that look like growths.

After the parasites: the wounds

Every attachment leaves a wound, and in poor water a wound is an invitation. Clean, stable water is the treatment for most of them, and the water testing guide sets out what to check. Watch each site for a week or two.

• A small red mark that fades is healing normally.
• A crater that deepens or spreads, with a white or grey edge, is becoming an ulcer and needs attention.
• A dark patch where the wound was is the skin repairing, and the black patches page explains why that pigment arrives after the damage rather than during it.
• Lifted or missing scales around an attachment regrow over weeks; the missing scales page covers the timescale.

Keeping them out

Every one of these parasites comes in from outside, which makes them among the most preventable problems in fishkeeping.

• Quarantine every new fish. At least four weeks, and six if the water is cool, in a separate tank: a louse takes 30 to 40 days to mature after hatching, and anchor-worm young develop very slowly in the cold, so a short or cool quarantine can miss them. The quarantine guide covers setting one up.
• Treat plants as a route in. Plants from another tank or a pond carry eggs and young as readily as fish do, and the aquascaping guide covers bringing them in safely.
• Be careful where live food comes from. Food netted from a pond that holds fish brings whatever lives in that pond, as the live food guide explains; cultured food from a clean source does not.
• Never move water, gravel or decor between a pond and a tank, or between tanks, without accepting what may come with it.
• In a pond, look at new fish closely before they go in, and look at the whole stock in late spring, when rising temperatures bring these parasites on. The pond guide covers the rest of pond health.