clams are ...
there are clams & mussels.
searching for info is ... frustrating
everything goes back to mussels due to the pearl industry
great we can turn our tanks into money makers
mussels are also known for having what's considered a parasite form during their youth where they attach to the gills of fish. ... i guess this is an evolutionary stage that provides the baby mussel with phytoplankton till it's old enough to have it's muscle systems developed properly ... as they grow they then leave the fish, ... that initial parasitic stage i think is little more than an irritation. if the fish is large enough i guess, it might prove dangerous to small fish. ... either way the warning was enough for me to say "not going there"
information on clams & mussels after that is sketchy
as henningc mentioned use greenwater to feed the clams (which i think that method is brilliant)
on the net feeding clams is "well they eat nitrates and/or ammonia" ... that's the majority of the answers found in the hobby, ... followed by your clams are doomed to starve over many months to a year, more searching can find the way to tell if your clams are doing well is measuring their size, if they are growing your doing good, if they are not growing, you may be in trouble.
and more searching, ... well in the hobby there are very very few sources, ... you could count them on one hand that said 'greenwater"
outside of the hobby, you can find phytoplankton easily.
feeding your bivalves is then easy 'greenwater'
in the hobby as i mentioned above is "dirty tanks weem to suport them easier and prstine clean tanks" (one of the search results i've found), ... found a couple that people are creating their own feed to spot feed them, syringe or turkey baster i think, ... the only one that comes to mind one person was feeding cooked cows blood or something (easy enough to get from your steak). ... i think that's due to a large unknown about how to take care of clams. ... easier to look at marine forums on taking care of clams...
that reminds me, i want to see if there are any photosynthetic freshwater clams

if there are they've gotta have nice colors

in coral tanks high nutrients are considered the death of coral though as it unbalances the symbiotic algae in the coral and one dies, then the other if nutrients are high, ... high nutrients are something of a given in freshwater
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otherwise, this thread, like many of my threads are full of curiosities of "what about this random seemingly crazy idea ? i want some input by more experience people"
usually when i run out of ideas to search for on my own and something spontaneously comes to mind
like DKRST's concern about removing the filter system (relating to my original curiosity)
henningc suggesting a sponge filter
-maybe one solves the concern for the other
but if henningc stayed on topic i wouldn't have had that insightful idea as an alternative
i'll be honest, i'm on the fence, but i've got more ideas
riskylight, ... what i've summarized above about taking care of clams is everything i have found from every source i have come across, ... clams in aquariums (especially freshwater) there is an extreme limit in information
and concerns of "can i smother clams if they are sitting in greenwater?" comes to mind, because there is so little information out there

their gills and feeding is all in the same organ area.
also due to the lack of knowledge about clams
talking with the supplier of my LFS about "how do you feed your clams?" "they eat nitrates", ... really, ... well so much for expecting healthy clams from them as they don't know how to take care of clams
the one clam i got died, ... i don't know if it arrived on the end of it's life, or if it was something going on in the tank, it was far too soon compared to what is said about clams starving to death is all i know, but even that doesn't mean squat