Reading Shell Health: Pitting, Erosion & What It's Telling You
A shell is a running record of the water it grew in. Here's how to read the marks it leaves behind — and what to actually do about them.
Pitting vs Erosion vs Growth Marks
Not every mark on a shell is a problem. Fine, regular growth lines are completely normal — they're just a record of growth spurts, same as tree rings. What you're actually watching for is different: pitting (small, rough, pockmarked patches, often near the newest growth at the shell edge) and erosion (a smoother, more general thinning or flaking, sometimes exposing a chalky or translucent patch underneath). Both point in the same direction — the snail isn't laying down shell material fast enough to keep up with wear, or existing shell is being broken down faster than it's replaced.
The Usual Cause: GH and KH
A shell is built almost entirely from calcium carbonate, and the two water parameters that govern how available that calcium actually is are GH (general hardness) and KH (carbonate hardness). Soft water — low GH — simply doesn't carry enough dissolved calcium for a snail to build shell efficiently, no matter how much food it's getting. Our GH & KH Explained guide covers what these numbers mean and how to read a test kit result properly if you haven't already got a baseline reading.
Low or unstable KH compounds the problem, because it's KH that buffers pH — a tank with no carbonate buffering swings more, and pH swings put additional stress on shell chemistry on top of a straightforward mineral shortfall.
A less obvious cause: low pH from unmanaged CO2 injection. If you're running CO2 for plants without keeping an eye on the knock-on pH drop, that acidity actively dissolves shell material — see our guide on CO2 in Invertebrate Tanks for how to run a planted, CO2-injected system without doing this.
What To Actually Do
Test GH and KH before changing anything else — guessing at a fix without a baseline reading tends to waste time and product. If GH is genuinely low, a remineralising supplement designed for freshwater invertebrates is the direct fix; our guide on Mineral Supported Systems covers dosing this properly without overshooting into water that's too hard for the rest of your stock.
One important thing pitting and erosion don't mean: they're not necessarily fatal, and they're not usually contagious in the sense a disease would be. A snail with a pitted shell can carry on feeding and reproducing perfectly normally once the underlying water chemistry is corrected — new growth at the shell edge will come in solid again, even if older, already-damaged shell doesn't fully repair itself.
- — Fine, even growth lines are normal — pitting and rough patches are the actual warning sign
- — Low GH is the most common cause; test before you dose anything
- — Unmanaged CO2 injection can drop pH enough to actively dissolve shell
- — Existing damage won't fully repair, but new growth comes in solid once parameters are corrected
Jack — Jack's Aquatics · Medway, Kent
UK Bred. Mineral Supported. Never Imported. — jacksaquatics.co.uk