What's the difference

Yeah, you can add both of them to the sump if you want, but the phytoplankton is usually used in small doses over a certain period of time. Don't dump the whole bottle/culture in all at once.
 
In a mature sump or a cycled refugium, copepods can reproduce and be self sustaining. Phytoplancton will be short lived and you will always be adding more. Some people keep a separate Phyto 'culture' going to feed the DT.
 
Many people don't use phyto at all: it can contribute phosphate, and copepods can get by on a cheato culture in the sump.
 
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