I use ESV Spray-Dried Phytoplankton. It is a powdered form of phytoplankton which is spray-dried to perserve the nutritional value of the food as opposed to freeze-dried methods which are, to my understanding, inferior.
It's easy to use and inexpensive. You simply mix the powder with RO or tank water and stick it in a blender for one minute. They recommend using it once a day with all fine
mechanical filtration removed.
I use it every other day.
From what I understand, phytoplankton is a very important additive as it is the basic building block of the complex food chain we know as plankton. Phytoplankton jump starts your food chain by providing the basic nutrition for zooplankton which is more readily used by corals and inverts.
I don't know how effective it is, but I can tell you that shortly after adding it to my tank, my open brain coral extends his tenticles out to feed.
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I'm no scrub...I let my ATS do all the work!