When replicating the ocean in an aquarium, there are initially no phytoplankton present in the water. The main reason for this is that the water used in our saltwater aquariums is filtered through a multi-filtering process, resulting in clean RODI water. Even ordinary tap water is chemically treated and would kill any living organisms within it. (If you’re interested, you can read this article on why you shouldn’t use tap water for your saltwater aquarium.)
If you have a fish only tank then their dietary needs are met when you feed them. But what if you have coral or other filter feeders? Examples of filter feeders are clams, sponges, oysters, etc.
Generally speaking, coral can survive without phytoplankton dosing, but you don’t just want coral to survive, you want them to thrive! Coral will be brighter, plumper and healthier looking. I great place to buy phytoplankton is from AlgaeBarn, available at Amazon.