They are bivalve and feed from the water column. If I skimmed every organic out of the water column the clam wouldn't be eating as well as it could, my plan is to not have to spot feed them. Just fyi I will the derasa and squamosa clams both of which are not the highest light requiring clams and can both reach 20 some odd inches or larger. I won't get rid skimming just cut to maybe a 12 or 16 hour skim time. Maximas and croceas are more light dependent (and brighter coloring), but I love the look of the big clams.