What daphnia are
Daphnia are tiny crustaceans called water fleas for the jerky way they swim. They filter algae and other fine particles out of the water. The larger Daphnia magna grows to about 5 mm and suits adult fish; smaller daphnia suit young fish. Shops sell them live or frozen; a culture at home gives a steady, clean supply.
Starting a culture
- Fill a tub or bucket of 5 gallons or more with dechlorinated water, or old tank water, and let it stand a day or two.
- Add a starter culture from a breeder, a fish club or a shop.
- Give them something to filter: green water from an algae-covered tank, or a small pinch of baker’s yeast or spirulina powder every few days, just enough to cloud the water faintly.
- Keep it in light shade, indoors or out, where it does not overheat.
Feeding the culture without crashing it
Too much food fouls the water and the whole culture can die in a day or two. Feed again only when the water has cleared. An air stone on a slow bubble, or a sponge filter, keeps the water fresher. Never let soap, insecticide sprays or other chemicals near it.
Harvesting and feeding
Net daphnia out with a fine net a few times a week, taking no more than about a third so the rest keep breeding. Rinse them in dechlorinated water and feed what your fish eat in a few minutes; leftovers live on in the tank until they are eaten. Daphnia from an outdoor tub can carry other pond life, so look the catch over before it goes in. The Merck Veterinary Manual names wild Daphnia and Cyclops as carriers of Camallanus, a red roundworm that infects guppies and other aquarium fish, so a culture started from a store or club starter is the safer source. How to spot Camallanus What fish eat