They will eat anything in the organic range of 5 microns or less. Try to vary the food as best as you can. There are some good powdered foods that have recently came out, but those have larger particals so do not directly feed it to the clam use as a general tank feed. Also when feeding Phyto place a 20 oz pop bottle over the clam by cutting off the bottom, unscrew the cap, place phyto in the bottle, replace cap, and leave for about 45 minutes.