Phytoplankton or algae culture.
We need algae cultures for our fry. The fry won´t see the rotifer in the raising tank easily without algae. The fry can estimate the strike distance of the prey (rotifer) better in phytoplankton.
Phytoplankton takes nutrients and waste products out of the raising tank.
It produces oxygen, which becomes lacking, if no phytoplankton is added.
Many larvae and many rotifer are swimming in a not circulated system. Every larvae and every rotifer is using the oxygen and produces carbon dioxide.
Phytoplankton provides a constant food source for the larvae´s prey. So we keep the nutrition value of the rotifer steady.
The most used algae is nannochloropsis (nanno).
Why?
Easy to grow, easy (more or less) available.
The nutrition value or better the fatty acid profile of nannochloropsis is really not ideal for fish larvae, but it works.
A mix of Isochrysis and Tetraselmis would be perfect, but both are not that easy to grow.
Many people are growing nanno in bottles. I don´t understand why. What can we do with that little amount of nanno?
I found this very practical. 20 litre, 60 bucks and it has got a tap.
I use skywhite (Osram) for light and mounted a mirror behind the tube. Photoperiod is 24/7. The fastest way to grow algae.
The biggest problem is contamination. That´s why this cannister is so useful.
Two holes in the lid, one for the airline and one to release the air we put in via airpump.
I put a few times the same dirty towel over the cannisters to make the room more dark. Two of my three algae cultures got eaten up by rotifer in no time. That always happens if you need algae the most. A rotifer egg must have fallen out of the towel, through the small hole into the cannister. But even via the air, that we pump into the cannister, many unwanted things can get in there. Ciliates, is a tiny zooplankton and comes through the air to eat your algae culture. Rotifer eggs as well.
That is the only way to go.
A 60ml syringe used as airfilter. The inlet and outlet is protected inside the syringe with a piece of foam. In between these two pieces of foam, I put activated charcoal. The cheapest charcoal I could find, because it has to keep the air dry. Nothing else.
That airpumps are really good. Two outlets, each of them connect to a syringe air filter and each feeds one cannister with air.
I use them same cannisters for rotifer. I will write more about it in new thread.
To feed 20 litre rotifer, you need 40 litre algae. Rotifer eats algae faster, than the algae can grow. I always use one algae culture until it´s finished. The rest algae, which you can´t get through the tap, I use as a start culture.
I have to write more about sterilization of sea water and the things I use at a later stage.
Also about how to feed the algae culture, etc.
, just kidding, its still a baby, it has replaced my mad scientist reef test lab, im ready to reef for REAL now 
