Photo: Tommy Kronkvist · CC BY-SA 3.0
Freshwater · Beginner · 10+ gal
Anentome helena
By Fishy Dave
Assassin snails (Anentome helena) are small striped snails that hunt other snails. Adults reach about 0.75 to 1 inch. One or two fit a 10-gallon, and three to five work on a pest snail problem in a 20 to 30 gallon tank over a few months. Keep them at 72 to 80°F in hard-ish water on sand. They breed slowly, one egg per capsule, so they don't overrun a tank.
| Water type | Freshwater |
|---|---|
| Care level | Beginner |
| Minimum tank | 10 gallons |
| Temperament | Snail hunters that leave fish alone. They eat bladder, ramshorn, and Malaysian trumpet snails, and they also kill nerites, young mystery snails, and other snails you meant to keep. Healthy adult shrimp are usually too quick for them. They spend much of the day buried in sand. Copper kills them. |
| Diet | Carnivore and scavenger. Live snails first, then leftover sinking food. Once the pest snails run out, feed sinking protein pellets, bug-based granules, or frozen bloodworms two or three times a week. They do not eat plants or algae. |
| Temperature | 72–80 °F |
| pH | 7–8 |
| Hardness / salinity | Moderately hard water, GH about 6 or higher. Soft, acidic water thins and pits the shell. Add crushed coral or a GH remineralizer if your tap or RO water is soft. |
The name oversells the speed. An assassin snail hunts at a snail's pace, and it still clears bladder and ramshorn snails out of a tank if you give it a few months.
Anentome helena comes from rivers and lakes in Southeast Asia, including Thailand, Malaysia, and Sumatra. Older books and shop tanks still call it Clea helena. The shell is a tall cone with yellow and dark brown bands, which is why some shops sell it as the bumblebee snail. Adults reach about 0.75 to 1 inch in aquariums. They have a trapdoor (operculum) to close the shell and a long siphon tube they wave around to smell food. Most live two to three years.
They are carnivores. In the wild they eat worms and other snails. In a tank they also scavenge dead fish and leftover sinking food, and they ignore plants and algae.
One or two fit a 10-gallon. For a tank overrun with bladder, ramshorn, or Malaysian trumpet snails, three to five assassins in a 20 to 30 gallon tank is the usual number. Expect a few months before the glass looks clean, not a few days.
Assassins work faster when you also feed less. Pest snails breed on food the fish leave behind, so cut feeding at the same time, trap them overnight, and pick off what you see. The pest snail guide has the full routine. A bigger crowd of assassins runs out of prey sooner, and then you are feeding all of them.
The calculator counts each assassin as a smaller waste load than a nerite. Check your own mix in the stocking calculator.
Keep them at 72 to 80°F with pH 7.0 to 8.0. In a cold room, an adjustable heater holds that range. Hardness matters for the shell. Aim for GH around 6 or higher, and test GH and KH with a test kit before you buy. Soft or acidic water pits and thins the shell, the same as with nerites and mystery snails. The water hardness guide covers crushed coral and remineralizers.
Assassins bury themselves in the substrate for much of the day and ambush snails from below, especially trumpet snails that live in the sand. An inch or two of fine sand suits them better than coarse gravel. The sand vs gravel guide covers the tradeoffs. Gravel works, but you will see them less and the babies have fewer places to hide.
Copper kills them. That includes copper snail killers, some ich medicines, and some plant fertilizers, so read the label before you dose anything into their tank. Finish the cycle before they go in, and drip-acclimate them over 45 to 60 minutes like any other invertebrate.
They eat almost any snail they can overpower. That covers every common pest snail and also the snails people buy on purpose. A nerite snail can close its trapdoor, but assassins still get into nerites, and small ones go first. A full-grown mystery snail is usually too big, but young mysteries are not, and they can kill a weak adult. If you want to keep either, keep assassins in a different tank.
Healthy adult shrimp move too fast to catch. Assassins will eat a shrimp that has died or is stuck mid-molt, and they sometimes catch baby cherry shrimp on the sand. A breeding colony may lose a few babies. Amano shrimp are too big for them.
| Tank mate | Risk |
|---|---|
| Bladder, ramshorn, and Malaysian trumpet snails | Eaten. These are the ones you want gone |
| Nerite snails | Often eaten, small ones first |
| Mystery snails | Babies and weak adults eaten. Healthy big adults usually left alone |
| Cherry shrimp | Adults usually safe. Some babies lost |
| Amano shrimp and fish | Left alone. Dead ones get scavenged |
When the snail supply runs out, assassins turn to the leftovers that reach the bottom. In a tank with cories or a pleco they often find enough. In a clean tank with no bottom feeders, drop a sinking protein pellet, a few bug-based granules, or a small piece of frozen bloodworm near the front two or three times a week. The sinking wafers page covers what reaches the bottom before the fish take it. Pull what's left after a few hours.
If you no longer want them, ask a local fish store to take them. Never release them into a pond or stream. They are a threat to native snails where they have been introduced.
Unlike bladder and ramshorn snails, assassins have separate sexes, and you can't tell males from females by looking. A group of five or six usually includes both. Pairs mate for hours, often at night.
The female lays small, clear, square capsules on glass, wood, or plant stems, one to a few at a time. Each capsule holds a single egg. In a lab study at 77°F the eggs hatched after about 52 days. The babies are about an eighth of an inch long and look like tiny adults. They stay buried in the sand for months, so you may not see them until they are about a third of an inch long.
Slow breeding is why they work as pest control. A tank only supports as many assassins as it has food for, so they never become the next snail problem.
A few listings we would open for this species. Skip anything you already own that still works.
Fluval Bug Bites Bottom Feeder granules, 1.6 oz
Small insect-based granules that sink fast and stay put on the sand. A pinch two or three times a week feeds assassins once the pest snails are gone.
Pull leftovers after a few hours. Food that sits overnight feeds the next round of bladder snails.
See this listingAPI GH & KH test kit
Shells need minerals. This liquid kit tells you whether your tap water is hard enough for snails or needs crushed coral.
If you already have an in-date GH/KH kit, you don't need another. A freshwater master kit still covers ammonia and nitrite.
See this listingFine aquarium sand, 20 lb
Assassins bury in the substrate and ambush trumpet snails from below. Fine sand lets them dig, and gives the babies cover.
Rinse it until the water runs mostly clear. An inch or two deep is enough for the snails.
See this listingOne or two in a 10-gallon. Three to five in a 20 to 30 gallon tank with a pest snail problem. Cut feeding at the same time, because the assassins work over months, not days.
Yes. They kill nerites, especially small ones, and baby or weak mystery snails. Assassins usually leave big healthy mystery snails alone, but there is no guarantee. Keep snails you value in a tank without assassins.
Healthy adult shrimp are too fast for them. They eat dead shrimp and can take shrimp stuck mid-molt, and a cherry shrimp colony may lose some babies.
No. They have separate sexes and lay one egg per capsule, and the eggs take about seven weeks to hatch. The food in the tank caps their numbers.
Leftover sinking food, frozen bloodworms, and sinking protein pellets. Feed a small amount two or three times a week in a tank with no other bottom feeders.
Next step
Plan on 10 gallons or more. Put your actual tank size into the stocking calculator and it shows what else can live alongside them.
Open the stocking calculatorBefore you buy
Keep your fishies swimming.
Freshwater · Beginner
Neritina / Clithon / Vittina (trade mix)
Nerite snails sold in shops are a trade mix of Neritina, Clithon, and Vittina. Zebra, olive, horn, and tiger are shell patterns with the same care. Beginner livestock if the water is hard enough. Adults reach about 0.75–1 inch. Plan one snail per 5–10 gallons, starting at 5 gallons. They graze algae and biofilm, lay eggs that do not hatch in freshwater, and need a lid.
Min. tank: 5 gal
Care notes →
Freshwater · Beginner
Pomacea diffusa
Mystery snails (Pomacea diffusa) are the gold, ivory, blue, and magenta apple snails sold for aquariums. Same care across those color morphs. Adults reach about 2 inches. Plan 5–10 gallons for one, and expect more bioload than a nerite. They breathe at the surface, lay pink clutches above the waterline, and will tour the carpet if you skip the lid. Hard water and calcium keep the shell intact.
Min. tank: 5 gal
Care notes →
Freshwater · Beginner
Neocaridina davidi
Cherry shrimp (Neocaridina davidi) are beginner invertebrates if the tank is cycled and copper-free. A starter colony of 10+ needs about 5 gallons; 10 gallons is easier. They need minerals for molting, not a second fish-page clone.
Min. tank: 5 gal
Care notes →
