Step one: be sure the snail is dead
Snails fake death convincingly. Mystery snails seal their trapdoor and sit motionless for days; nerites wedge into a corner and sulk; a snail floating at the surface may simply be riding an air bubble in its shell. Removing a live snail on suspicion is a real risk, so confirm first. Lift the snail out and smell it — this is the oldest test in fishkeeping because it is infallible. A dead snail smells powerfully, unmistakably rotten, a smell you will not confuse with anything else in the hobby. A live one smells of nothing much. Supporting signs: a dead snail's body hangs slack from the shell or the trapdoor sits loose and sunken, while a live snail's trapdoor is held firmly shut and the body retracts if touched gently.
Step two: get it out and keep it out of the water
Once you are certain, net or lift the snail out promptly — promptness is the whole point, because decay starts immediately and a snail is a dense parcel of protein for its size. Drop it straight into a bag and into the household bin; do not leave it on the tank rim while you fetch things, and wash your hands afterwards. If the shell is a handsome one you would like to keep, know that the remains must be fully cleaned out first, and many keepers simply let the shell go — an incompletely cleaned shell returned to the tank is the same pollution problem wearing a disguise.
Step three: test the water the same day
A decaying snail releases ammonia as it breaks down, and in a small tank even one mystery snail can register on a test within a day. So test, do not guess: run an ammonia test as soon as the snail is out, and a nitrite test too if the death was not fresh. Any ammonia reading above zero means act — a partial water change with temperature-matched, dechlorinated water, sized to the reading, then a retest later the same day to confirm the level is falling. Also check the filter is running normally and give the substrate a light siphon where the snail lay, since that patch received the worst of the decay.
Step four: check the rest of the livestock
One dead snail is usually an individual story — age, in a group of snails bought together, is the commonest explanation of all — but it earns a headcount. Look over the other snails for the same slackness, check fish for clamped fins or unusual hiding, and think back over the last few days: any new additions, a forgotten feeding binge, a filter switched off and forgotten. If a second snail is dead or fish look wrong alongside a rising ammonia reading, treat it as a tank problem rather than a snail problem, keep testing and changing water daily, and hold off feeding for a day while levels settle.
Step five: decide whether anything needs replacing
Resist the urge to immediately buy a replacement and equally the urge to strip the tank down. If tests come back clean and everyone else is well, the correct response was the removal and the check you have already done. Write the test results and the date into the tank's records, so a repeat event shows a pattern instead of a blur. When the next motionless snail leaves you unsure whether it is resting or gone, put a follow-up question to KinsLe's Kimmy; she answers using the KinsLe Answers library.