05-26-2009, 01:12 PM
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The earlier replies have answered you, and I would strongly suggest you not add shells. You haven't indicated the pH and hardness of your tank water, and by comparison the tap water, so it is difficult to assess the extent of the effect. But one thing is certain, the shells will dissolve, adding salt (even if washed this can still occur) and calcium, and depending upon the water volume and size/number of shells, the pH will rise as will the hardness. If you have livebearers or rift lake cichlids, this will not be a problem. But if you have acidic water fish, it is certainly the last thing you want. And, if the salinity or pH changes enough, fish stress occurs and that leads to other health problems and even behavioural issues. Fish are trapped in your aquarium water, and anything that alters it is going to have an effect on the fish internally.