Aquarium pests and hitchhikers: what to worry about

What you see: very thin white, tan or pinkish threads, a few millimetres to a couple of centimetres long, wriggling in a whip-like motion. Usually in the substrate, often noticed in a cloud after you disturb the gravel, or occasionally swimming in the water column.

What they are: harmless segmented worms that live in the substrate eating waste and leftover food. Almost every established tank has them, most keepers just never see them.

What to do: nothing, medically speaking. Large numbers in open water mean the substrate is holding a lot of organic waste, so treat them as a maintenance prompt rather than a pest: feed a little less, and vacuum the substrate more thoroughly at water changes. They will retreat back out of sight. Many fish will happily eat them.

Do not confuse with: Camallanus worms, which protrude as short red threads from a fish's vent and are a genuine parasite. See our common fish diseases guide. Detritus worms are in the substrate, never attached to a fish.

What you see: tiny white or grey specks, around a millimetre, on the glass and among plants. Copepods dart in sharp jerky hops. Seed shrimp (ostracods) look like miniature grains of rice or sesame seeds and scuttle steadily rather than hopping.

What they are: microscopic crustaceans grazing on biofilm and detritus. Their presence is a sign of a mature, biologically active tank, and they are free live food. Fry in particular hunt them enthusiastically.

What to do: nothing. If the numbers become visually distracting, the cause is surplus food, so feed less and they will thin out. They cannot harm fish, shrimp or plants.

What you see: small shrimp-like creatures up to about a centimetre, flattened side to side rather than top to bottom, scurrying through the substrate and along surfaces, often on their sides.

What they are: freshwater amphipods, usually arriving on plants. They are scavengers that eat detritus, leftover food and decaying plant matter, and they are excellent live food for anything big enough to catch them.

What to do: generally nothing. In very large numbers they may graze on soft or already-damaged plants, and they compete with shrimp for food, so shrimp keepers sometimes thin them out. Reducing feeding is the control. They do not attack healthy fish or adult shrimp.

What you see: small white or translucent worms, usually under 5 mm, gliding smoothly across the glass and plant leaves. Crucially, the head end is rounded or blunt.

What they are: a harmless flatworm that grazes biofilm. They are extremely common in shrimp tanks and planted tanks, and they are the single most misidentified organism in the hobby.

What to do: nothing at all. They do not eat shrimp, eggs, fish or plants. Before you treat anything for planaria, read the next section, because the treatment is far more dangerous to your tank than these worms are.

What you see: a small pale stalk, typically 2 to 10 mm, anchored to the glass, a leaf or the hardscape, with several fine tentacles waving from the free end. They stay put and contract into a blob when disturbed.

What they are: tiny relatives of jellyfish and anemones. They are stinging predators, but they can only catch things smaller than themselves.

Do they matter? It depends entirely on your tank. In a normal community tank of adult fish, hydra are a curiosity and harm nothing. In a shrimp tank or a breeding tank they are a genuine problem, because they catch and eat newly hatched shrimplets and small fry (see keeping freshwater shrimp).

What to do: hydra bloom on surplus food, so cutting feeding right back removes what feeds them and populations often collapse by themselves. Scrape any you can see off the glass and siphon them out during water changes. Some fish, particularly gouramis and bettas, will pick at them. Chemical treatments exist but carry real risk to snails, so read the treatment section below before reaching for one.

What you see: a flatworm gliding smoothly over glass and substrate, commonly 5 to 20 mm, and the identifying feature is the head: a distinct triangular or arrow-shaped head with two visible eyespots that give it a cross-eyed look.

What they are: predatory and scavenging flatworms. They hunt slowly, and they will take fish eggs, weakened or moulting shrimp, shrimplets and small snails. They do not attack healthy adult fish.

Do they matter? In a fish-only community tank, honestly not much: they signal excess food and little else. In a shrimp tank they are a serious pest, because a moulting shrimp is defenceless and shrimplets are easy prey.

What to do: cut feeding hard, since planaria populations are driven by surplus food. Trap them, which works well: a small bottle or purpose-made trap baited with a piece of raw prawn or bloodworm, left in overnight and removed in the morning, takes out large numbers with no chemicals. Repeat over several nights. Only if trapping fails should you consider a chemical treatment, and the section below explains why that needs care.

What you see: a squat, six-legged, drab brown or green insect larva, roughly 1 to 4 cm, usually sitting very still among plants or on the substrate. Damselfly nymphs are slimmer with three feathery tails; dragonfly nymphs are bulkier and blunt-ended. Both are pictured on the cards above, and the tails are the quickest way to tell them apart.

What they are: the aquatic juvenile stage of dragonflies and damselflies, and they are ambush predators with an extendable hinged jaw they fire out to seize prey. They will kill shrimp and small fish, including fish considerably larger than you would expect.

How they arrive: almost always on plants collected from a pond or waterway, or in water from an outdoor pond. They cannot appear in a tank stocked entirely from a shop.

What to do: remove them manually, straight away. Net or trap them out the moment you see one, and check thoroughly for others, as where there is one there are usually more. There is no practical chemical control that would not also harm your shrimp. This is the most common reason for fish and shrimp mysteriously vanishing from a tank containing wild-collected plants.

What you see: a segmented worm that moves with a distinctive looping, inch-worm motion and can stretch and contract dramatically. Usually brown, grey or black, and often found on the glass or under decor.

What they are: most leeches that turn up in aquariums are scavengers on detritus and snails rather than fish parasites, so the picture is less dire than the name suggests. Some species do feed on fish, however, and telling them apart is not realistic for a hobbyist.

What to do: remove them manually and treat the tank as needing a thorough clean and inspection. If you find one attached to a fish, do not pull it off forcefully, as that can leave mouthparts in the wound and cause a worse infection. Consult our diseases guide and treat the resulting wound. Like nymphs, leeches almost always arrive with wild-collected plants, wood or pond water.

Everything else on this page lives in the tank. If the organism is on the fish, the category changes from hitchhiker to parasite, and it needs treating rather than tolerating.

Fish lice are flat, disc-shaped, greenish crustaceans up to about 5 mm, visible moving across a fish's flanks. Anchor worm appears as a fine white or greenish thread protruding from a raised or reddened sore, with a forked end. Camallanus worms show as short red threads at the vent. All three are treatable, but none will resolve on its own, and all are covered in our common fish diseases guide.

Fish flicking against surfaces, clamped fins, or unexplained sores warrant a close look even if you cannot see anything attached.