This is kind of wide spectrum question. "Brains" feed off of many different things depending on what kind of brain it is. Some of them with only a singular mouth will eat just about anything they can pull in over a period of days; while others that use longer polyps to capture particles, will take in smaller micron sized food. In a lot of my previous tanks, I would dice up PE Mysis shrimp (high nutritional value) and use a large ended syringe to squirt them directly. Phytoplankton feeds a wide variety of things, just not that much towards brains (look at the difference in nutritional value between phytoplankton and a can of Cyclop-Eeze). Cyclop-Eeze and Coral Frenzy are two of my favorite foods.
You can take an empty CLEAN salad dressing bottle (this works for me anyways), fill it with tank water and several of tablespoons of each. Store it in the refrigerator, and shake it once a day for 3 days. This makes the particles heavier so that they do not float to the top. After the food has absorbed water, I find it helpfull to fill an ice cube tray with the mix and stick it in the freezer. Then I feed one cube per 35 gallons of water about 5 times a week.