Planaria, hydra and Scutariella: shrimp tank pests and what to do

Most of what gets called a pest in a shrimp tank is harmless, or a population that grew because more food goes in than the shrimp eat. Very little needs a chemical, and most damage comes from keepers who identify the wrong organism and reach for a fish medication. Pests also sit low in the ranked causes of shrimp deaths.

Quick identification

What you see	Likely organism	Real risk
Flat, 2–20 mm, triangular head, glides	Planaria	Real, to shrimplets and weak adults
Flat, under 3 mm, blunt front, darts	Rhabdocoel flatworm	Low
Stalked polyp with tentacles, contracts	Hydra (Hydra spp.)	None to adults; contested for young
White 1–3 mm threads on the rostrum (the shrimp's nose spike)	Scutariella japonica	Low; treatment optional
White fuzz on rostrum, legs or shell	Vorticella	A symptom, rarely the cause
Round threads in the substrate	Detritus worms (segmented) or nematodes (hair-thin)	None
Tiny 2–4 mm cone clamped to glass or leaves	Freshwater limpet (Ferrissia)	None
0.5–2 mm beans scooting jerkily over the substrate	Seed shrimp (ostracods)	None

Planaria

Freshwater planaria are free-living flatworms in the order Tricladida, in aquaria usually Dugesia or Girardia: flattened, off-white to grey-brown, 2–20 mm. The tells are a broadly triangular head with two small eyespots and a smooth glide over a mucus trail, driven by cilia (microscopic beating hairs) rather than muscle. Thrashing or whipping usually means a detritus worm or a nematode, though a planarian can undulate as it swims. The look-alike rhabdocoel flatworm is under 3 mm, blunt in front, and generally harmless.

Planaria are predators and scavengers; predation on small invertebrates and eggs is well established for the group. The risk falls on newly released shrimplets (baby shrimp), dropped eggs, and adults already compromised: freshly moulted and soft, or dying of something else. A healthy adult is not on the menu, and shrimplet loss from planaria is poorly quantified. They regenerate from fragments, so scraping one in half can leave two.

The population runs on surplus protein and detritus (sunken waste debris), so most infestations are really an overfeeding problem — see feeding shrimp: what, how much, how often — and cut the feed before anything else. Then trap them: a baited jar left on the substrate at night, lifted out in the morning.

Chemically, the usual choice is an areca nut extract — the seed of Areca catechu, sold in the hobby as betel nut — marketed as No Planaria and similar. It reportedly works at label dose without killing shrimp, but the margin between effective and harmful is poorly documented, with scattered reports of losses in Caridina. It also kills snails; siphon the die-off and run several small water changes, not one large one.

Treatments that kill shrimp

Shrimp are far more sensitive to aquarium medications than the fish those were made for, and there is no hospital tank option: whatever you dose, you dose the animals. Caridina — bee shrimp (Caridina cantonensis), crystals (usually placed in C. logemanni, though that genus assignment is unsettled) and tigers (C. mariae) — are widely held to be the less forgiving group.

• Copper medications. Shrimp need trace copper. It sits at the centre of haemocyanin, the copper-based protein that carries their oxygen, and it has a role in moulting, so leave it in their food and in a correctly dosed plant fertiliser. The hazard is dose. Copper sulphate, chelated copper and the copper in many fish and snail treatments are lethal at concentrations fish tolerate, and free copper measurable in the water column will kill a colony. "Safe for scaleless fish" does not mean safe for invertebrates, and no safe chelated-copper level for shrimp is well established.
• Dewormers. Fenbendazole (Panacur) is the classic hobby answer for hydra and planaria. It kills both, and snails with them. Its safety for shrimp is genuinely contested: many keepers report no losses, others a colony gone. Praziquantel is more often called well tolerated, but reports on how reliably it clears planaria are mixed.
• Hydrogen peroxide spot dosing. Used against hydra, with quoted doses spanning a wide range and no agreement on a safe one.

Hydra

Hydra (Hydra spp.) are small freshwater cnidarians (the jellyfish and anemone family). They sit still, attached by a basal disc to glass or plants: a body column of roughly 2–15 mm when extended, tentacles several times that length, contracting to a barely visible blob when disturbed, though the quoted size range varies between species and sources. They hunt with stinging cells (nematocysts), taking small prey that blunders in: rotifers, copepods, Daphnia — all tiny water creatures. Adult shrimp are far too large to be at risk. That hydra take newly released shrimplets is standard hobby wisdom, plausible on prey size but poorly documented.

A bloom is a food-chain signal, not contamination: surplus food grows the microfauna hydra eat, and cutting the feed usually collapses the population. Siphon rather than scrape, since hydra also regenerate from fragments. We ship unsexed juveniles (young shrimp) at 0.6–1.2 cm, exactly that size class, so clear a visible population before releasing new stock.

Scutariella japonica

Scutariella japonica is neither a worm nor a leech: it is a flatworm in the order Temnocephalida, family Scutariellidae, an ectosymbiont (an organism that lives harmlessly on the outside of another) of freshwater atyid shrimp (the dwarf-shrimp family) — most often Caridina, though Neocaridina davidi carry it too. It shows as white, 1–3 mm translucent threads on the rostrum, moving with a looping, leech-like action; often the eggs show first, as tiny white capsules beside them.

They are generally described as feeding on bacteria and organic particles around the gill region rather than on the shrimp — closer to commensal (a harmless lodger) than parasitic. Whether a heavy load interferes with gill function is contested: some keepers treat on sight, others keep infested colonies breeding for years.

The standard response is a brief salt dip, though no recipe is authoritative: around 20–30 g of non-iodised salt per litre of dip water for 30 to 60 seconds is commonly quoted, and the eggs reportedly survive it. Mix the salt into tank water so temperature and pH match, dip one shrimp first and watch it throughout, then move it to clean water and discard the dip. Never dip a shrimp soft from a recent moult, or one not yet settled after arriving — see acclimatising new shrimp.

Doing nothing is defensible: an ectosymbiont on an active shrimp is not a defect. Arriving dead is another matter, and our DOA and missing items policy sets out the terms: claim within 8 hours of delivery, with photos and a short video of the animals in the unopened bag, and the remedy is replacement or a refund of the livestock value, your choice.

Vorticella

Vorticella are stalked ciliates: single-celled suspension feeders, a bell on a contractile stalk that coils like a spring. On a shrimp they read as white fuzz and are routinely mistaken for fungus — clear under magnification, often not by eye. They are not eating the shrimp; they filter bacteria, so heavy growth means a high organic load, or a shrimp too unwell to groom. Reduce organics and siphon, using small frequent water changes rather than one large one. Colonised shell is shed at the next moult; struggling to moult is the more urgent problem, covered in moulting problems and the white ring of death.

Detritus worms and nematodes

Detritus worms are aquatic oligochaetes (relatives of earthworms), usually Naididae: segmented, round in cross-section, 5–20 mm, thrashing when they leave the substrate. Free-living nematodes are the other common sighting — hair-thin, unsegmented, a few millimetres long, whipping against the glass in a stiff S. Both graze detritus and bacteria, and neither is thought to threaten shrimp, though nematode parasitism of freshwater shrimp is not well documented. A swarm on the glass at night just means more organic matter than the system is processing.

Limpets and seed shrimp

Two more hitchhikers get reported as pests and are really messengers. Freshwater limpets (usually Ferrissia species) are tiny cone-shaped grazers clamped to glass and leaves; seed shrimp (ostracods) are little bean-shaped crustaceans scooting over the substrate inside a hinged shell. Neither harms shrimp of any age — and both populations run on the same fuel: overfeeding, debris, and dissolved organics in the water. A bloom of either is the tank telling you more is going in than the system can process.

So the first treatment is the usual one: feed less, siphon more. Chemicals are off the table twice over here — snail treatments are copper-based, and a seed shrimp is a crustacean, so anything that kills one is a hazard to the animals you are keeping.

Biological control works if you want them gone faster. Small shrimp-friendly fish and their fry will graze seed shrimp out over time. Limpets are harder — most fish ignore them, and while bottom-dwellers are worth a try, reports are thin. The one certainty is the mini puffer (Carinotetraodon travancoricus, sold as the pea or dwarf puffer): it will clean out limpets and seed shrimp within a few days. It is friendly to adult shrimp but can potentially take shrimplets, so treat it as a short-stay cleanup crew rather than a resident in a breeding tank.

Keeping them out

Nearly all of it arrives on plants, hardscape or water from another tank. Tissue-culture plants are the cleanest start; anything else earns a quarantine of three to four weeks, longer if you can, so cocoons and snail eggs hatch out of reach. Bleach and alum dips are faster but hard on the plants, and their effect on planarian cocoons is not well established — the dip and quarantine routine is in best plants for a shrimp tank. A tub of plants needs no heater; any tub holding shrimp does, because an unheated container in an Australian late winter tracks room temperature, and cold shock will hurt a colony long before a hitchhiker does.

The honest first treatment is rarely a bottle: feed less, siphon more.