Assassin Snail Care Guide

The assassin snail is the one snail you buy on purpose to kill other snails — a carnivorous hunter that ambushes and eats pest snails (and ornamental ones too), which is exactly why it must never share a tank with snails you want to keep. Like every aquarium snail it needs hard, calcium-rich water or its shell erodes.

Assassin Snail at a glance

The sourced figures the welfare engine uses to judge Assassin Snail — the parseable key facts.

Where it comes from

Assassin snails are Anentome helena (von dem Busch, 1847), long sold under the older name Clea helena, in the family Nassariidae — a mostly marine family of scavenging dog whelks, of which the assassin is a rare freshwater offshoot. They come from freshwater rivers, streams, canals and lakes across Southeast Asia, especially Thailand, Malaysia and Indonesia (including Lake Toba on Sumatra), where they burrow into soft, sandy or muddy beds and ambush prey from below. Two things about that origin drive the care. First, like all snails in the clade Neogastropoda the assassin is a carnivore and scavenger — it does not touch algae or plants, so it is no clean-up crew. Second, its native water is mineral-rich, which is why the aquarium target is hard, neutral-to-alkaline water with plenty of calcium for the shell. One honest accuracy note: a 2017 DNA study found that the trade name hides a complex of at least four near-identical species, so two assassins from different shops that do not quite match in banding are probably different cryptic species, not a bad batch.

Did you know?

• It is a snail you buy specifically to kill snails — the hobby's go-to biological control for a bladder or ramshorn outbreak.
• It ambushes from the sand with a snorkel, raising a siphon to "smell" passing prey before striking with an extensible proboscis — marine-snail hunting behaviour in a freshwater tank.
• It is a marine snail's freshwater cousin, a rare freshwater member of Nassariidae, the family of scavenging dog whelks.
• "Assassin snail" is probably several species: a 2017 DNA study found the trade name Anentome helena hides a complex of at least four near-identical cryptic species.
• It breeds in freshwater but lays one egg per capsule, so unlike pest snails that dump clutches it multiplies slowly and never overruns the tank.
• The familiar name Clea helena is outdated; the accepted current name is Anentome helena.

Tank size — and why

Around 5 US gallons is the practical floor for a single snail, with 10 gallons better for a small group or an active pest-control role; Aquarium Co-Op notes one snail can live in as little as 2 gallons, but small volumes swing in parameters. The constraint is not swimming room or territory — this is a slow benthic snail with no group need — it is a steady prey or protein supply plus stable, cycled water. Stocking is driven by how much food there is, not by any social requirement.

As a guide, a 20-gallon tank comfortably suits about 9–13 Assassin Snail as a single-species display, leaving room for tankmates.

Water parameters in practice

Hard water and calcium are the load-bearing welfare fact, exactly as for every aquarium snail. The shell is calcium carbonate — snail shells run roughly 95 to 99.9 percent CaCO3 — and in soft, acidic water it pits, develops holes and slowly dissolves, shortening the snail's life. If you see holes or pitting in the shell, suspect soft water. Aim for neutral-to-alkaline pH of about 7.0 to 8.0 (tolerated up to ~8.5, but never persistently below ~6.5 or the shell erodes), GH of roughly 8 to 15 dGH, and KH of about 5 to 15 dKH; supplement with crushed coral, cuttlebone, a mineral block or calcium-rich snail food where your tap water is soft. Temperature sits comfortably around 22 to 27 °C, and these are tropical animals that tolerate the warm end better than the engine's conservative ceiling suggests. Copper is lethal to all inverts, ammonia and nitrite must be zero, and the snail dislikes rapid swings, so drip-acclimate after shipping.

Will it thrive in your water?

The comfortable range for Assassin Snail is about 18–28 °C (64–82 °F) and pH 6.5–8. Test your own tap water against it below.

These are the sourced comfortable ranges. Stable water matters more than chasing an exact number — a steady reading inside the band beats a "perfect" one that drifts. Some fish also need a particular water hardness (GH); where that applies, the prose above covers it.

Diet & feeding

A strict carnivore and scavenger. Its whole purpose is eating other snails — bladder, pond, ramshorn and Malaysian trumpet pests, plus their eggs — at a rate of roughly one pest snail every couple of days per assassin, and it will also take carrion such as dead fish or leftover protein. It does not eat algae or live plants. The single biggest post-purchase killer follows directly from this: once the assassin has cleared the pest-snail outbreak it was bought for, its food source is gone, and a buyer who assumes "a snail that eats snails needs no feeding" will watch it slowly starve. After the job is done you must switch to feeding protein — thawed bloodworms, brine shrimp or mysis, sinking carnivore pellets or Repashy-type gels — about once or twice a week.

Gear & setup

Sand or fine gravel a couple of inches deep is a genuine requirement, not décor: the assassin burrows to ambush prey, and the juveniles live buried for their first months, so a bare-bottom or coarse-rock-only tank denies its natural behaviour and its breeding. Rock and driftwood give it hard surfaces to glue egg capsules to and are fine, but they do not replace the sand. It is far less of an escape risk than a nerite — it is a burrower, not a climber — but a lid is still sensible, and moderate, well-oxygenated flow suits it without being a hard requirement.

Temperament & behaviour

Two facts have to be stated together: it is peaceful toward fish, plants and healthy shrimp, and it is a relentless predator toward snails. It has no interest in or ability to harm fish, but it hunts and kills all other snails — pest and prized alike — and multiple assassins will gang up to overwhelm a larger snail such as a mystery or nerite. This is why it must not be kept with ornamental snails you want to live, and why those snails are vetoed from its tankmate suggestions. It hunts by burying in the sand and raising a siphon like a snorkel to "smell" passing prey, then extending a proboscis to consume the soft body, leaving the empty shell behind.

Group & social needs

Solitary, with no group requirement — it neither shoals nor needs company, and there is no intraspecific aggression, so multiples coexist peacefully (and cooperate to overwhelm large prey). Keep one for spot pest control or several for a heavier infestation, driven entirely by how much prey or food is available. The one caveat: because the sexes are separate and cannot be told apart by eye, a single snail cannot breed, so if you actually want reproduction you need a group of around six or more to be statistically sure of both sexes.

Compatible tank mates (preview)

A short, engine-cleared shortlist — the species TankStocking's welfare engine clears with Assassin Snail and that suit its size and temperament best. Tap a name for its care guide, or use + to load the pairing in the planner.

• Black Neon Tetra+Uses the midwater zone, peaceful temperament, similar adult size
• Black Phantom Tetra+Uses the midwater zone, peaceful temperament, similar adult size
• Cardinal Tetra+Uses the midwater zone, peaceful temperament, similar adult size
• Celestial Pearl Danio+Uses the midwater zone, peaceful temperament, similar adult size
• Cherry Shrimp+Peaceful temperament, similar adult size
• Chili Rasbora+Uses the midwater zone, peaceful temperament, similar adult size
• Clown Killifish+Uses the top/surface zone, peaceful temperament, similar adult size
• Dwarf Emerald Rasbora+Uses the midwater zone, peaceful temperament, similar adult size

This engine-cleared shortlist is Assassin Snail's tankmate surface for now — a dedicated tank-mates guide can follow for high-demand species.

Breeding & sexing

Easy to keep, slow to breed. Assassins are gonochoristic — separate males and females with no reliable external difference — so you simply keep several to get a breeding group. The headline contrast with nerites is that the assassin completes its whole life cycle in freshwater, yet it reproduces so slowly that you will never wake to a tank overrun: a pair locks together for around twelve hours to mate, then the female lays single eggs, each in its own small translucent capsule glued to hardscape, glass, wood or plants — one egg per capsule, which is why it multiplies gently rather than in clutches. Eggs hatch after roughly three to eight weeks, the hatchlings burrow into the sand and live buried for two to three months, and they reach maturity around six months.

Lifespan

Typically about 2 to 3 years, with up to roughly 5 years possible under excellent care in stable, mineral-rich water. What shortens it: soft or acidic water eroding the shell, copper exposure, ammonia or nitrite from an uncycled tank, rough acclimation of wild-collected stock, and — the leading avoidable cause — starvation once the pest snails are gone and no protein is offered.

Common mistakes

• Keeping it with snails you love — it eats nerite, mystery and rabbit snails and ganks up on larger ones. Don't mix it with any snail you want to keep.
• Expecting it to clean algae — it is a strict carnivore and does nothing for algae or plants.
• Not feeding it once the pest snails are gone — the number-one post-purchase death; switch to protein once or twice a week.
• A soft, acidic blackwater tank — the calcium-carbonate shell pits and dissolves; supplement calcium or keep it elsewhere.
• A bare-bottom or coarse-gravel-only tank — it needs soft sand to burrow and ambush, and juveniles develop buried.
• Housing it with snail predators — loaches (clown, yoyo), pufferfish, large cichlids, goldfish or crayfish will eat or crush it.
• Any copper in meds, algaecides or ferts — read every label; copper kills inverts.
• Expecting a giant — it stays about 2 to 2.5 cm; the "3 inch / 7.5 cm" figure on some blogs is wrong.

Signs of trouble

• A pitted, holed or colour-faded shell, worst at the spire — soft, acidic water dissolving the calcium carbonate; raise hardness and add calcium.
• Empty pest-snail shells appearing around the tank — this is the snail working as intended, not a problem.
• A sealed, buried snail that stays shut — it may simply be resting or hunting, so confirm death by smell before removing, and remove promptly to avoid an ammonia spike.
• Assassins dying off weeks after a pest-snail outbreak is cleared — almost always starvation; start feeding protein.
• Lethargy or loss of the usual burrowing and hunting after a parameter swing or copper exposure.

Is this fish right for you?

Don't buy an assassin snail if you keep ornamental snails you want to live — it hunts and eats nerite, mystery and rabbit snails and gangs up on the big ones, so it is incompatible with a snail collection. Don't buy it expecting an algae or clean-up animal; it is a strict carnivore. Skip it if your water is soft and acidic and you won't supplement calcium, because the shell erodes, and don't house it with loaches, puffers, large cichlids, goldfish or crayfish that will eat it. There are no dyed or deformed morphs to avoid here, but stock is largely wild-collected from Southeast Asia, so quarantine and observe new arrivals — wild Anentome can carry trematode metacercariae — and drip-acclimate gently.

Bringing one home

Drip-acclimate slowly after shipping — assassins dislike sudden swings in pH, hardness or temperature, and are highly copper-sensitive. Stock is mostly wild-collected, so observe and quarantine new arrivals (wild specimens can host trematode metacercariae), add them only to a mature, cycled tank with sand to burrow into, and never use copper-based treatments.

Common questions

Will assassin snails eat my nerite or mystery snails?

Yes. Assassins hunt and eat other snails, including prized nerite, mystery and rabbit snails, and several will gang up to overwhelm a larger one. Don't keep them with any snail you want to keep — that's why ornamental snails are dropped from their tankmate suggestions.

Do assassin snails eat shrimp?

They leave healthy, active adult shrimp alone — a slow snail can't catch one. But they will opportunistically take moulting, sick, weak or very small shrimp and occasionally shrimplets, so keep them well-fed and give shrimplets dense cover if you're breeding shrimp.

Do assassin snails eat algae or plants?

No. They are strict carnivores, so they won't clean algae and won't damage plants. If you want an algae-eater, an assassin is the wrong animal.

Will assassin snails overrun my tank?

No. Although they breed in freshwater, they lay single eggs one capsule at a time and reproduce slowly, so they rarely become a pest themselves — one of the main reasons they're used for snail control.

What do I feed assassin snails once the pest snails are gone?

Protein, once or twice a week — thawed bloodworms, brine shrimp or mysis, sinking carnivore pellets, or Repashy-type gel food. Forgetting this is the most common reason assassins die after they've done their job.

Do assassin snails need hard water?

Yes — hard, neutral-to-alkaline water with calcium. The shell is calcium carbonate and erodes in soft, acidic water, so aim for pH about 7.0 to 8.0 with meaningful GH and KH, and add crushed coral or cuttlebone if your water is soft.

Sources & confidence

Sources & confidence (9 species)

These back the Assassin Snail figures and the previewed tank mates above. Each figure is read from the TankStocking species database (v2026.06); below is the care reference behind it and how confident we are in that data. Confidence reflects the source quality, not whether any pairing is safe. Full source list and the welfare model are on the methodology page.

• Assassin Snail Anentome helena (Clea helena) — The Shrimp Farm (theshrimpfarm.com/posts/assassin-snail-care) high confidence
• Black Neon Tetra Hyphessobrycon herbertaxelrodi — Seriously Fish / Aqua-Fish (Hyphessobrycon herbertaxelrodi) high confidence
• Black Phantom Tetra Hyphessobrycon megalopterus — Seriously Fish (Hyphessobrycon megalopterus) high confidence
• Cardinal Tetra Paracheirodon axelrodi — Seriously Fish (seriouslyfish.com/species/paracheirodon-axelrodi) high confidence
• Celestial Pearl Danio Celestichthys margaritatus — Seriously Fish (seriouslyfish.com/species/celestichthys-margaritatus) high confidence
• Cherry Shrimp Neocaridina davidi — Aquarium Co-Op cherry shrimp care; The Shrimp Farm high confidence
• Chili Rasbora Boraras brigittae — Seriously Fish (seriouslyfish.com/species/boraras-brigittae) high confidence
• Clown Killifish Epiplatys annulatus — Seriously Fish (Epiplatys annulatus); Aquarium Co-Op high confidence
• Dwarf Emerald Rasbora Celestichthys erythromicron — Seriously Fish (seriouslyfish.com/species/celestichthys-erythromicron) high confidence

Care-guide sources (8)

This guide synthesises the references below; where they disagree, the range and the disagreement are noted in the text above. The figures in the key-facts box are read from the TankStocking species database (v2026.06). Full welfare model on the methodology page.

• Wikipedia — Anentome helena — authority (von dem Busch, 1847), synonyms (Clea helena, Canidia helena), family Nassariidae, SE-Asia/Lake Toba range, size 18-32 mm, carnivore (worms + gastropods), ambush burrowing, single egg per ~1-1.5 mm capsule
• Strong, Galindo & Kantor (2017) — Quid est Clea helena? (PeerJ 5:e3638) — reinstates genus Anentome, places it in Nassariidae (sister to Nassodonta), concludes the trade A. helena is a complex of at least four cryptic species; US trade sample conspecific with Singapore stock
• Aquarium Co-Op — Assassin Snail care — size ~1 in, 1 snail in ~2 gal+, temp 21-27 °C, pH 7.2-8.0, moderate-hard GH, strict carnivore (supplement bloodworms/pellets/Repashy), shell pits in soft water -> crushed coral, separate sexes, slow single-egg reproduction, shrimp-safe
• Superior Shrimp & Aquatics — Assassin Snail (Anentome helena) Care & Breeding — size 2.5-3 cm, lifespan up to ~5 yr, 10 gal optimal / 5 gal min, sand >=2 in for burrowing, temp 24-28 °C ideal (18-30 tol), pH 7.0-8.0, GH 8-20, KH 5-15, shells 95-99.9% CaCO3, ~1 snail/2 days, gonochoristic, never overrun, siphon-snorkel ambush, copper lethal, predators
• aquariumbreeder — Do Assassin Snails Eat Shrimp? — cannot catch a healthy shrimp; takes only moulting/sick/weak shrimp; shrimplets rarely; shrimp eggs safe (carried under tail); keep assassins well-fed and give cover
• Aquarium Tidings — Assassin Snails (Clea helena) care & breeding — diet (snails/worms/carrion), pairs lock ~12 h, single eggs in square capsules, hatch 3-8 weeks, juveniles burrow then mature ~6 months, >=6 to ensure both sexes, sand best, gang up on larger snails
• Chomchoei, Strong et al. (2018) — cryptic diversity & echinostome metacercariae in Anentome helena — corroborates cryptic diversity; reports wild Anentome hosting trematode metacercariae (quarantine note)
• MolluscaBase — Anentome helena (von dem Busch, 1847) — accepted-name authority and synonymy

More on Assassin Snail

Related guides on TankStocking — each scored by the same welfare engine as the planner.

This care guide is a sourced planning reference, not veterinary advice — individual fish, filtration and maintenance all matter. Cycle the tank, test your water, and observe your fish. How TankStocking works →