Clean and sterilize the flask thoroughly using liquid sanitizer or bleach to ensure no malevolent bacteria are in it. Follow product manufacturer's instructions for the proper water to sanitizer ratio.
Fill up the flask one half to three quarters full with sea water. The amount of sea water you will need will be dependent on the size of your flask and the amount of Dunaliella tertiolecta you are trying to grow. Be sure to have an exact measurement of the amount of sea water used.
Add your Dunaliella tertiolecta culture. Your mixture should be 5% Dunaliella tertiolecta and 95% sea water. The amount you use again depends on the size of your flask.
Add one part fluorine for every five parts Dunaliella tertiolecta added to the mixture. The fluorine will act as a nutrient for the algae, enabling them to feed and multiply.
Plug the opening of the flask loosely with cotton, then place aluminum foil with air holes poked into it over the lid.
Place the flask outdoors where it will receive as much direct sunlight as possible for up to 20 days. At between 15 and 20 days, the algae will have multiplied to its maximum population.