The Need for Phytoplankton in Saltwater Aquariums
Phytoplankton are basically the microscopic plant-like organisms, photosynthetic creatures that drift about in the ocean and absorb the sunlight turning it into delicious sugars.  Phytoplankton happens to be key for your saltwater tanks looking at the fact that in the end the filter feeding invertebrates such as the clams, corals and sponges will get to eat on them for their nutrition.  Phytoplankton have been billed a good source of nutrition for your marine life looking at the fact that they have a generally high level of concentration in Omega 3 unsaturated fatty acids which happens to be one of the key nutritional needs in a number of the invertebrates.
Talking of the phytoplankton strains that you can grow at home for your saltwater tanks, there are three basic and most common kinds.  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 is the particular phytoplankton strain that is largely seen to be the best for the fact that it happens to be well balanced, having such better levels of concentration of the essential fatty acids, HUFAs and EPA, which are more than essential for the growth of the fishes at their larval stage of development.  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.  Over and above this, Nannochloropsis has as well been deemed to be a good fit for saltwater aquariums for the fact that it has some antibacterial properties. Read on  Saltwater Aquarium Supply
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.  Besides this in as much as the strains will be as hardy to grow afterwards, there will be a need for you to spare some time monitoring and for maintenance of the culture set up on a daily basis.  Instead of going through all the stress there comes with culturing your own strains of phytoplankton for your need to dose your saltwater aquarium, you can alternatively settle for the many available commercial products for the same purpose and avoid all the inconveniences there may be with the option of culturing your own.
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. View  https://www.youtube.com/watch?v=JHY_o4fVIf0