Eggs are commonly available, as soon as you get to a certain number of tanks you end up buying a one pound can. The eggs must be kept dry and are hatched in aerated natural or artificial seawater. Attempts to use rock salt and a few other chemicals to simulate seawater don't work well. They lack the trace elements required and over time this can lead to a nutritional deficiency. This may not show overtly, but the point it they'd look better and be healthier if the eggs were hatched properly.

The nutritional value of the BBS is only significant upon hatching - the yolk sac is everything and this is used up in a couple of hours, so, when you read "freshly hatched brine shrimp" they mean "freshly hatched brine shrimp".