Ah, the green stuff. The miracle cure, the lifeblood of a reeftank, the savior of fan worms and clams of all ages... Or maybe not so much. Could it be that phytoplankton actually does more harm than good in a reef aquarium?
In the ocean there is a lot of phytoplankton and it's crucial to the ecosystem. In the ocean. In your tank, however, phytoplankton dies and fouls up your water quality. I mean let's think about it, you're pouring live green stuff into your tank where chances are that only a miniscule part of it will be used for food. In fact, it is arguable if any realistic part of it will be used for food by your tank's inhabitants.
To lay to rest some of the myths: fanworms do not need phytoplankton, neither do pods, tubeworms, your rock, or clams. Or anything. It's quite possible, and by quite possible I might mean a 100% certainty (refer to disclaimer below), that nothing at all in your tank will notice the difference as to whether phytoplankton is present or not. In my system I haven't been using phytoplankton from day one, and I have walls of fanworms and tubeworms, tons of pods and mysid shrimp to the point that even my morbidly obese mandarins can't seem to eat enough to make a dent, and very tiny 1" maxima clams that grow 1/8-1/4" every few weeks with light alone.
There have been some questionable studies done, funded in part by phytoplankton growing companies, that make it seem like all small clams need phytoplankton to survive, and these studies are based on the fact that small clams (under 2") will consume phytoplankton for energy whereas larger clams tend not to. While it is true that phytoplankton is beneficial to some extent for very small clams, it has been shown that it is not essential for keeping small clams. Or any clams at all! (Editor's note: legitimate research has shown that Tridacnid spp. clams merely use the phytoplankton, as a nitrogen source and as we all know, our aquariums usually have an ample supply of nitrogenous products available!)
The fact of the matter is that very small clams, like any very small organism, do not adapt well to being moved, bagged, shipped, and acclimated. Unfortunately they have been appearing more and more in this hobby due to clam farms in the indo-pacific having very little supply of larger clams, and simply shipping out the juveniles at nearly the same prices.
Regardless, when small clams die it is often blamed on their lack of phytoplankton, which is in part erroneous since acclimation and shipping stress was the original issue. It has been shown that very small clams will do better when removed from the tank and placed in a bowl with a high amount of phytoplankton and sea water, called "target feeding", but it has also been shown that small clams will do well with light alone being that they already contain the zooxanthellae needed for photosynthetic growth. The more you read up on it, the more you'll notice that for every argument for phytoplankton there are about a dozen much better arguments for not putting it into your tank.
Unless you specifically plan on keeping a lot of small clams and target feeding them daily, which would be a bad idea in and of itself in my opinion, you can do without phytoplankton with no ill effects. When you go to a fish store though expect to hear that your tank will need it or it will not do well. The simple fact is that phytoplankton can and will compromise your water quality, leading to increased algae, more nutrients, slow or stopped coral growth due to increased phosphates, and definitely loss of coral color. Save your money for more productive things and don't let your LFS sell you things you don't need, this hobby's expensive enough already!