Raising Daphnia


Daphnia are small aquatic crustaceans that live in fresh water. The tropical fish books often mention daphnia as a food for fish. I have never seen daphnia for sale in pet shops. You either have to collect them in the wild or raise your own. Collecting them in the wild is not convenient for most of us, and I would be concerned with introducing fish diseases or parasites if the collection site has fish. Fortunately raising your own daphnia is very easy if you have a little space.

There are a number of daphnia species. Daphnia pullex and Daphnia magna are the most common ones in North America. Most species are the size of a small pin head, but Magna get up to about 1/8”.  Don’t get hung up with which type you have. I think I have at least two different species in my daphnia ponds. The larger magna type tends to appear in the cooler spring and fall months, and smaller varieties appear during the warmer months.  

Daphnia are often called water fleas because they are sort of shaped like fleas, and swim with a jerky motion. They are filter feeders, eating suspended algae and other microscopic swimming organisms.   

Daphnia Pool


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.   

You can let the pools freeze during the winter. The wading pools seem to handle this fine. In the spring they will thaw out and the eggs will hatch. Here in southern Wisconsin the daphnia reappear in late April and I can harvest until about the end of October.   

A couple of years ago I made a little “green house” to cover the in-ground pool. This keeps it warmer and I am getting about another month at each end of the season.  A couple of years ago we had a hard freeze in mid December. The other pools  were frozen solid, but the covered one only had about 1/2" of ice. That thawed in a couple of days and I kept harvesting until mid-January when typical Wisconsin winter weather returned.

This cover adds about a month to each end of the growing season to the wading pool set into the ground. It acts like a little green house, keeping the water warm.  If I were to do it over again, I would make the center board taller so that snow and water would run off better. 

The cover is made of 2" X 4" lumber and covered with plastic.

Raising Daphnia Inside

Some people raise daphnia inside during the winter.  I have done this, but am not sure if it is worth the effort and cost, especially if you need to have lights to produce green water.  One method which produces enough for occasional feeding requires a south facing window but does not require additional lighting.   

I use a gallon pickle jar to raise green water.  See the article on raising green water.  To raise the daphnia I use either one gallon jars or 2 liter soda bottles. Fill the container up about ½ full with green water. Fill the rest with clean water up to the point where the surface area is decreased. Add about a dozen daphnia and put them in the sunny window.  

In about 3 weeks the water will be clear, hopefully replaced by a large number of daphnia. Pour the water through a fine fish net. Pick out a dozen or so of the larger ones to start the next bottle and feed the rest to your tropical fish.   

The nutritional benefits of daphnia have been debated.  They are mostly water. The fish love them, and seem to grow and condition for spawning when they are fed, so I think it is worthwhile raising daphnia.   

You don’t have to worry about over feeding with daphnia.  They will live in the tank until eaten.  Sometimes I add a few large daphnia into a tank with fry at least ¼” long.  The daphnia are far too big for the fry to eat, but they will be able to eat the baby daphnia that are produced. 

© 2009 - 2022 Gary C. Sutcliffe


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