Assassin Snail

Despite the name, the assassin snail (Anentome helena) is a snail, not a fish, and it earns that name by hunting. This small Southeast Asian species, native to Thailand, Malaysia, and Sumatra, grows to around 2.5–3 cm and wears a pointed, bumblebee-striped shell. It spends much of the day buried in sand with only its breathing siphon poking up, sampling the water for the scent of prey. The moment it smells food, it surfaces and goes hunting.

What it hunts is other snails. Assassins eat bladder, ramshorn, and Malaysian trumpet snails, which makes them the go-to fix for a pest-snail outbreak, with no chemicals needed. They will also scavenge carrion, fish eggs, and fry too young to swim away. They ignore plants and leave dwarf shrimp and peaceful fish alone, but they do pick off ornamental snails like nerites and mystery snails, so don't mix them with snails you want to keep. Skip puffers and snail-eating loaches, which turn the hunter into the hunted.

Care is easy. A single snail is fine in around 8 litres, and a small 20-litre tank easily holds a group, at 21–27 °C with a slightly alkaline pH of 7.2–8.0 and moderate-to-hard water. Soft water pits their shells, so add crushed coral if yours is low in minerals. When the pest snails run out, feed high-protein foods like frozen bloodworms, sinking wafers, or pellets. One quirk sets them apart from most aquarium snails: they have separate sexes and can't clone themselves, so keep six or more to get a breeding pair. Females lay single eggs in tiny, square, translucent capsules stuck to rock or wood. These hatch within a few weeks as miniature snails, with no free-swimming larval stage, and they breed slowly enough that your tank will never be overrun.