You should not need to feed them. Even if you use a DeNitrator there will be enough nitrates for them. They feed from the sand and not the water column so you can't really measure what they are getting.
they dont just consume nitrates they are filter feeders, they will consume planktonic foods, and anything small enough for them to consider food. in an established tank there is really no reason to attempt supplemental foods for them.
good luck even finding a cleaner clam in your tank to feed it...usually they will bury themselves in substrate and you will forget about them until your parameter start climbing due to a dead clam decaying
Originally Posted by HatesSushi
When a clam dies the shell opens all the way so that all the little crabs can come in and eat it.
True. If you look closely one of them always brings the Tobasco.