as mentioned by helter, the easiest way to use brineshrimp for baby fish is to hatch them out from eggs. To grow up brineshrimp you need a good size container (200+ litres) filled with salt water. Then you either add some fertiliser and light and wait until the water goes green and soupy. Or you can add things like liquid fry, but you need a lot of it.
The birnehsrimp eggs can be hatched in a 1litre container with about 1/2litre of saltwater in. You only need a small amount (1/4 level teaspoon) of dry brineshrimp eggs for this amount of water. Sprinkle the eggs over the water and then add an airstone to the container. Allow it to bubble away for a day or so. When you see the water change from brown to orange, the eggs have hatched. Remove the airstone and wait 10minutes. the newly hatched brineshrimp (known as nauplii) will gather in a group. Use an eye dropper to suck them out and feed to the baby fish.
You can also get decapsulated brineshrimp eggs that have the shell removed. These are hatched in the same way but take less time and don't have shells to worry about, (baby fish can't eat the egg shell and sometimes choke if they swallow them).
