What kind of plankton ?
I feed a few brands of "frozen plankton" that are basically a small krill (E. pacifica instead of E. superba) -- slightly bigger than a mysids shrimp IME. I feed these to my fw and marine fish that are too small for full sized krill, its a great color enhancer and source of HUFA's in my opinion.
(I use the Hikari, but have also used the San Fran Bay brand )