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, because acid DOES attack
the calcium salts directly, so you would do well to read the list of
ingredients before using such products on a precious specimen.
 
Paul M.