i had a bunch of freshwater clams in my 29 gallon for a year or so, and they did ok, beside puffer attackking them occasionally. also, they didnt reproduce at all for some reason, but they had trouble burrowing in my substrate, i guess because it wasnt sand like they're used to. BUT they all died over time, i think because my house got very cold during the winter (my tank was heated of course) but the water did get a little colder than normal. anyway, i didnt lose any fish during the time i had them, but its still a possibility