Talking of phytoplankton, these are basically microscopic plant-like organisms which are photosynthetic creatures that drift about in the ocean absorbing the sunlight and turning it into delicious sugars. Looking at their essence in your saltwater tank, the filter feeding invertebrates such as the sponges, corals and clams will finally feed on the phytoplankton for their nutrition. Generally speaking, phytoplankton has been deemed a sure source of good nutrition for marine life for the fact that they happen to contain such high concentration of the Omega-3 unsaturated fatty acids which are a dietary requirement for the majority if not all of the invertebrate marine life.
You will basically have three most common strains of the phytoplankton that you will be able to culture at home for your saltwater tanks. These are the Nannochloropsis, Isochrysis, and Tetraselmis. Looking at all these strains, the one that happens to be most common and popular is Nannochloropsis. This particular phytoplankton strain is seen to be such a well balanced strain with relatively higher levels of the fatty acids, HUFAs and EPA, which are quite essential for the growth of the larval fishes. For this reason if at all you are considering the breeding of saltwater fishes in your aquarium, then this is the best strain that you will be advised to start with. Further to this is the fact that Nannochloropsis has as well been seen to have some antibacterial properties in it.
You can choose to culture or not to culture your own phytoplankton anyway. But all in all you need to appreciate the fact that it actually takes some time and a sure investment for you to get to set up your culture station and as well prep the stuff to grow. Moreover, even though the phytoplankton strains mentioned above have the hardiness to continue the growth on their own after then, you will need to ensure that you have spared some time on a daily basis for you to maintain the culture set up or else you run the risk of seeing the culture crash. Alternatively to growing your own and suffering all the inconvenience of maintenance and or risk of crashing, you can make use of the available commercial phytoplankton strains and dose your saltwater tank with these instead.
Note the fact that many aquarists do dose phytoplankton into their aquariums as a supplement to the feeding schedule given the fact that they happen to be so natural and nutritious food sources for marine life.