When you hatch the BBS, the shells float to the surface with the airstone which bubbles inside. So when you harvest them, you harvest them from the bottom of the water. There is a very simple DIY hatchery made from 2 liter pop bottles. The instructions are easy to follow, and it allows for you to harvest safely from the bottom. The packet of BBS should say how much to add per batch. There should be multiple batches in the packet. To begin with, you only feed freshly hatched shrimp. That means one hatching per day. Which is why you want 2 hatcheries going.

The liquifry and first bites are both excellent. The next food should be the BBS, they take a day or two to hatch, so you would want to start right away. After that the liquifry or the First Bites. Hopefully you have the BBS close by so you can get started. And don't worry, making the hatchery is not as complicated as it looks.

Perhaps Chickadee can give you an opinion of how long you can keep the fry on egg yolk. Usually, you would start the BBS or other food at day 5.