I can give some cons to em:
Clams are obnoxiously sensitive to water quality
They need very clean water, and wont tolerate ammonia, or hard metals like copper as much as fish can tolerate them.
They need good water flow
They do best in large bodies of water such as Koi ponds
I had a river clam once (found it in my local river) when i was little, but regardless of how unexperienced or unknowledgeable i was, it still died the next day lol. I used river water too. Hell i cant even remember what they eat.
Didnt you used to have a dwarf frog? i thought i saw it in your sig once...thats kinda exotic in my opinion. There's crayfish, but they also have very specific keeping requirements.
You know how you can learn more about clams? Search around for Koi pond info. they are often sold with the likes of frogs and snails as additions to Koi ponds.