Sorry, but I'd also discourage you from introducing clams.
Ryan had it right when he said their contribution would be significantly less than some healty growing plants.
Years ago, before I knew better, I got one (the "Golden Clam" which is able to withstand tropical temperatures) and it lasted a few months. But WHAT a mess when it died! It really messed up the water, and the horrible smell as I tried to get it out of the tank was something I can still recall.
Plus, once it got itself mostly buried in the substrate, there wasn't much to see.
If you're really interested in a filter feeder, check out the bamboo shrimp. They'll perch near the filter output (ie, in a strong current) and put out their fans to catch debris. A bit of pre-wet powdered fry food dripped in front of them can keep them fed, as they might not get enough food from the filtered water from the output.
-Jane
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