This has been a decades old question with 2 schools of thought: Infusoria ( green water) vs newly hatched Brine Shrimp. Personally, I use a live plant/ lettuce base for nest building which creates a small culture of infusoria right into the tank. After 1 day of the free swimming fry eating this, I start using newly hatched brine shrimp. My thoughts for doing this are since the larger fry generally are big enough for newly hatched brine shrimp from the beginning of the free swimming stage, the smaller fry ( which may be too small for this larger food) will have a choice of foods to eat and grow on.
Bottom line, I've had many larger quantities of fry by doing this "multi food" method even though I also have had decent sized amounts of fry with just doing the newly hatched brine shrimp alone. So dealer's choice on feeding
Hope this helps