Descripción
The bladder snail is the one almost nobody buys and almost everyone ends up with. It arrives as a stowaway among the leaves of some new plants, as a tiny egg nobody sees, and within weeks you have five.
It differs from other small snails in one curious detail: its shell is coiled to the left. Almost every snail in the world is right-coiled; the bladder snail is sinistral, and if you hold the shell tip up, the opening faces left. It is also noticeably faster than the rest: for a snail, it practically runs.
It is not a pest, it is an indicator. This is the shift in perspective worth making. The bladder snail eats organic debris, leftover food and dead leaves: it can only multiply if there is a surplus. An exploding population is not a snail problem, it is a warning that you are feeding more than the tank consumes.
It also does useful work removing organic matter that would otherwise break down and raise nitrate.
If you still want to reduce it: adjust the ration and within weeks the population drops on its own. Snail poisons are the worst possible option, because they contain copper and wipe out any shrimp in the tank. A piece of blanched courgette left overnight works well as a trap: in the morning you lift it out with twenty snails aboard.
Hábitat
Physella acuta is cosmopolitan, and that is its whole secret. Native to North America, it is now present in shallow fresh waters on five continents: streams, irrigation channels, urban ponds and any body of water with some organic matter.
It survives conditions that would kill other snails: temperatures from 10 to 30 degrees, poorly oxygenated water, moderate pollution and widely varying hardness. It is so adaptable that it is used as a bioindicator organism in water quality studies.
That tolerance explains why it turns up in your tank even though you never bought it: it travels on imported plants, survives transport and acclimatisation, and finds any aquarium habitable.
And it also explains why it is so hard to eradicate completely once established. The good news is that, understood for what it is — an opportunistic cleaner whose population reflects the surplus — there is no need to eradicate it at all.