The best containers I have found for raising daphnia are the small plastic children’s wading pools about 3-4’ in diameter. They provide a good surface area, are inexpensive (~$8) and last several years.
I have several. One is in full sun and is buried into the ground. The others just sit on the ground and have partial shade. Generally more sun is better since that produces more algae on which the daphnia feed. Daphnia tend to go through cycles where you have a bloom and more than you can use, followed by a crash where you only can see a few in your pool. Having ones in different conditions of sun and shade helps to ensure you will have at least one pool producing. The buried one produces earlier in the year and lasts longer into the fall than the other ones.
Fill the pool with water, add a little soluble plant fertilizer like Miracle Grow, and some green water. If you don’t have green water just add some aquarium water. In about a week the water will start to turn green. At that point add in your starter culture.
If conditions are right, you will have a bloom in 2-3 weeks. You can harvest them with a fine fish net. Put them in a white plastic dish with water and examine for dragon fly larvae, water beetles and other predators that might get into the pool. I have not had many problems with these, but they do show up occasionally. You want to keep these pests out of your fish tanks. I then use a turkey baster to feed the tropical fish in my aquariums. One nice thing about Daphnia is that if you overfeed the fish, the extra daphnia will continue to swim around the aquarium until the fish are hungry again.
From time to time you might also collect other critters like mosquito larvae, glass worms, or blood worms. That is great! A little variety for dinner! Some people are concerned that the daphnia pool will become a major mosquito breeding ground. This has not been a problem for me. I sometimes get a few mosquito larvae in the early spring, but not that many. I try to collect all of them both to feed the fish and to keep the adult mosquito population down. Once the daphnia get going, the mosquitoes become uncommon. Either the daphina keep the water too clean and the female mosquitoes won't lay eggs there, or the daphnia eat the newly hatched mosquito larvae.
As the daphnia population grows, the water will become clear and the daphnia population will decrease. Supplemental feeding will help keep the daphnia reproducing. I like to keep a 5 gallon plastic pail in the sun to grow green water. About once a week I distribute about 2/3 of the green water in the pail between the pools. The bucket is refilled with clean water and another pinch of Miracle Grow is added.
You can supplement feeding with some pea flour or diluted squash baby food if you don’t have green water. Don’t add much though as you could pollute the water and kill everything off.
If you just leave the pool alone there will be periods when you don’t have enough to collect. Eventually conditions will improve and you will get another bloom of daphnia.
Daphnia have two ways of reproducing. Normally they reproduce parthenogenically. Females give live birth to female offspring without the benefit of male fertilization when conditions are good. When conditions are not good, typically in the fall when the water starts to cool, they start producing males. The females mate with the males and produce egg packets. These look like black knap sacks on their backs. They release these and you can see them floating on the surface when it starts to get cold.