A healthy shell is smooth, solid and still growing
Start with the newest shell first, the band right behind the opening where the snail is actively building. In suitable water that growing edge is smooth and even, the same thickness all the way around, and it extends the shell's colour and pattern without a pale, crumbly rim. Work backwards over the whole shell: the surface should feel and look glossy rather than chalky, the spiral should run unbroken to a tip that is rounded or neatly formed, and the colour should stay consistent instead of fading into washed-out patches. Press very gently with a fingertip — a sound shell is unyielding. A snail that has lived in good water carries a shell like armour it happens to live in; a snail that has not carries the evidence on its back where you can read it before the animal itself looks unwell.
The damage unsuitable water leaves behind
Water that is too soft or too acidic literally dissolves a shell, and the pattern of the damage tells the story. Look for pitting — tiny craters scattered across the older shell — white, rough patches where the outer layer has worn through, thinning you can see as dull, lighter areas, and erosion at the very tip of the spiral, which is the oldest and longest-exposed part. Cracks, a flaking surface, and a growing edge that comes in thin, distorted or stopped altogether all say the snail cannot get or cannot use enough mineral to build with. Distinguish old scars from an ongoing problem by the new growth: damage behind a clean, strong new band is history from previous keeping, while fresh pitting or a ragged edge forming now means the water in front of you today is the water doing it.
What suitable water actually provides
A shell is built from calcium, so suitable water for most aquarium snails such as mystery and nerite snails means water that is not soft and acidic: a neutral to alkaline pH and a reasonable general and carbonate hardness give the snail both the raw material in the water and, with a suitable diet, in its food. That is the whole mechanism — in acid, soft water the shell dissolves faster than the snail can repair it, while in harder, alkaline water building outpaces wear. Test the tank rather than guessing from the shell alone, and check the trend after water changes, because a tank filled with soft tap water can drift more acidic as it ages. Mineral-rich foods and a calcium source in the tank support shell building from the inside while the water protects it from the outside, and both halves matter for a growing juvenile most of all.
Acting on what the shell tells you
If the shell shows active erosion, adjust the water gradually — raise hardness and stabilise pH in small steps over days, not in one large correction, because the snail inside the shell dislikes sudden change as much as the shell dislikes acid. Then watch the growing edge as your report card: within a few weeks a strong, smooth new band should be forming if the water now suits the snail. A shell that keeps deteriorating, a snail that stays withdrawn, stops eating or shows damage to the body at the opening needs an aquatic vet or an experienced aquatic shop's assessment rather than more home adjustment. Keeping dated shell photographs beside the water readings in KinsLe in the Pet Passport turns a vague impression that the shell looks worse into a documented change anyone helping you can judge at a glance.