Keeping Dead (empty) shells in an Aquarium

(Original Forum Question : I recently purchased 5 Golden Cowries just to use as aquarium decorations; will they deteriorate over time in my aquarium?)

They will deteriorate over time, and the rate at which they deteriorate will be affected by the pH (acidity/alkalinity) of the water. Some kinds of aquarium fish require slightly acidic conditions (pH about 6.5). Others require slightly alkaline conditions (pH about 7.5), while others prefer a neutral pH (7.0). If your tank is on the alkaline side of neutral, deterioration will be relatively slow. But if your tank is on the acidic side of neutral, not only will the shells deteriorate faster, they will also neutralize the water in the tank as they dissolve into it. Coral will have the same effect.
(Answer by M. Paul Monfils via the Forum)