IMO/IME feeding phytoplankton is not a requirement for keeping clams long term. I've kept Tridacna clams for years in a tank without being regularly fed phytoplankton. In my opinion, feeding plankton of the right size and in the right density does benefit clams, though.
I would have plankton available for food when bringing new clams into a system, myself. Being moved seems to weaken clams and feeding them helps them overcome the stress, IMO. If you want to forego a live phytoplankton setup, IME DT's refriderated plankton is a good clam food. I kept a 3" squamosa for months in a 10G quarantine tank under 32W of PC light by feeding DT's, so I am pretty confident it is effective food for clams. That clam is over 9" today. You do need to buy the DT's from a reliable source as dealers who don't know or don't care can easily spoil the DT's by mishandling. DT's also looses food value relatively quickly so don't purchase more than you can use in a few weeks. Another feeding option for clams is refridgerated yeasts in liquid, which are available from Knopp as clam/reef food and from Aquatic Eco System as rotifer food. I can't say that I've sustained clams on yeast, but I have fed my system quite of bit of yeast over the years with no apparent negative effect. One more clam food is cryopreserved plankton. While some people are dubious that a frozen plankton retains its integrity, I note that cryopreserved planktons are produced for commercial aquaculture, so I would think it must be effective if handled and used properly.
All that said, my opinion is that a live phytoplankton culture will yield better clam food than stuff from a bottle. But is phytoplankton essential - no - not unless the clam is small and/or weakened, IMO.
