When it comes to clams cleaning your tank they consume nitrate. People with clams in their tanks often have no detectable nitrate at all. We had a clam expert come to our club and he said the fastest growing clams he had were in a holding tank his workers urinated in (don't try this at home). As far as food for them if they are less than 3 inches they need phytoplankton to survive. When they get above 3 inches they can survive mainly off of strong light (MH is recomended). Someone will tell you that you can get away with VHO or PC for clams but if you want to give them the best possible conditions go MH! Be carefeul with the puffer he may nip at clams!