Sodium hypochlorite will not harm well calcified shells, but on long exposure may harm poorly calcified shells like Haminoea, Akera, Anomia, some of the more delicate land snails, and some brachiopods. This is because, even though the compound does not affect the actual calcium salts of the shell, it does attack the organic matrix that is often present in small amounts between the layers of calcium salt crystals. Therefore, the higher the ratio of organic matrix to calcium salts, the greater is the potential for noticeable damage. Sodium hydroxide (or potassium hydroxide) ("lye") may well be found in these cleaners as well. These substances are similar to hypochlorite in their reactions with organic matter. However, the hypochlorite content of Chlorox and similar products is usually about 5% or less. The hydroxide content in liquid toilet bowl cleaners and drain cleaners can be considerably higher, and their action on organic materials proportionately more vigorous. And of course if you buy these cleaners in crystalline (dry) form, you could make the solution as strong as you want to. Better to dilute such products substantially first, and try the weaker solution, to see if it will do the job, before trying stronger solutions. Also, remember that the term "organic substances" includes your skin - enough said. My main concern about toilet bowl cleaners and other products intended for use on porcelain is that they often contain phosphoric acid. ANY acid is detrimental to ANY shell, so you would do well to read the list of ingredients before using such products on a precious specimen. Paul M.