Descripción
The ramshorn is instantly recognisable because its shell does not form a tower like almost every other snail, but a flat spiral, coiled in a single plane like a rolled rope. It grows to 3 cm and comes in red, brown and bluish varieties.
It also has a biological oddity worth telling: its blood contains haemoglobin, like ours, and that is where the reddish colour of the pale varieties comes from. Most molluscs use haemocyanin, which is bluish and carries oxygen less efficiently. That haemoglobin lets it live in very oxygen-poor water where other snails could not.
In the aquarium it is a tireless cleaner: it eats organic debris, leftover food, dead leaves and algae off the glass, without touching healthy plants.
On its reputation as a pest. It does breed readily, but its population is limited by one thing only: available food. If you have dozens of ramshorns, you do not have a snail problem, you have an overfeeding problem. They are simply making use of what is left over.
That is why snail poisons are a bad idea: they kill the consequence, not the cause, and they wipe out the tank's shrimp too. Adjust the ration and the population regulates itself within weeks.
Hábitat
Planorbarius corneus is a European snail, which sets it apart from almost everything else in a tropical aquarium. It lives in ponds, canals and lakes with still, heavily vegetated water across much of the continent, Spain included.
That temperate origin gives it a very wide thermal tolerance: it copes with anything from 15 to 28 degrees without concern, so it works equally well in a heated tropical tank and an unheated cool-water one. It is one of the few invertebrates that suits a temperate aquarium.
Like all snails it needs medium to high hardness to maintain its shell. In soft water the same symptoms appear as in nerites: erosion, porosity and holes at the apex.
And a warning if you are considering it for an outdoor pond: precisely because it is European and hardy, never release it into a natural watercourse. Even where it is native across much of Europe, moving populations around transports parasites and pathogens.