DT's, or other live phytoplankton.
You can start a culture from these but check the date on the bottle the newer batches are easier to culture
Any downside?
When culturing algae you add a fertilizer F/2 Algae Feed,Micro Algae Grow or Miracle Grow Houseplant liquid food.
When you add algae from your grow out to your tank, you also add this fertilizer which can cause unwanted algae growth.
There is a lot of discussion on what benefits from algae addition in your tank. You could add it and get no benefit.
Algae is used as a food for other organism (Artemia,Copepods and Rotifers). These are filter before adding to the
tank so you don't have the fertilizer
The biggest downside is it a lot of work with little benefit. If you are breeding fish or have difficult fish than
it can be worth it but many just use the Algae concentrates ex. (These are just examples you can find these products at many vendors)
http://www.aquaticeco.com/subcategories/1370/Concentrated-Planktonic-Feed
For people growing Algae a group buy of F/2 Algae Feed would be the best deal.
http://www.aquaticeco.com/subcategories/1364/ProLine-F-2-Algae-Feed/algae/0