i dont make my own phyto. i dont even add phyto to my tanks. if you have strong lighting and feed your fish regularly, theres no need to add phyto just for the clam. phyto is just a source of N & P that is passed onto the zoox, so in most home aquariums with well feed fish there is plenty of N & P. if you still want to feed phyto thats fine, just add it directly to the tank and start slowly.
heres a study where they wanted to figure out how the clams acquired zoox and moved them into the zoox tubular system. they took larval clams and kept them in micro filtered water so they couldnt get any particulate. these clams never received any phyto and did just fine on light alone.
http://links.jstor.org/sici?sici=000...2-N&size=LARGE 
