The Fishroom Diaries- Make Room for Daphnia

January 12th, 2017

By Rob Crosby

All experienced goldfish breeders are familiar with the challenge of keeping young fry adequately fed with live food throughout the first 6-8 weeks of life. Newly hatched brine shrimp (sometimes referred to as bbs for short), are the usual favorite amongst breeders as a first food, and an essential part of the breeding process. However, relying solely on brine shrimp has its drawbacks too; optimum brine shrimp hatching requires a 24 hour incubation at 82°F, and eggs that have been stored for a few months may take even longer to hatch. As well, bbs are short lived in fresh water and any uneaten bbs can quickly foul the water and jeopardize the fry. Many breeders have unpredictable schedules or travel occasionally, so this time away from the fish room can mean that fry must go without during their absence and possibly up to 24 hours after returning. A reliable “backup” live food option can be a real lifesaver in situations like this.

Veil fry feeding in a daphnia cloud

If you’ve ever wondered how commercial Chinese and Japanese goldfish farms were able to produce large numbers of well-grown goldfish long before brine shrimp eggs were available to them, the answer is daphnia. Daphnia are a genus of small, 0.2–5 millimeters in length, freshwater planktonic crustaceans, and they have the remarkable ability to reproduce asexually, giving birth to live clones of themselves. Feeding primarily on coliform bacteria and unicellular algae, their numbers will grow exponentially until the available nutrient source is exhausted.

Traditionally, goldfish farmers would prepare daphnia ponds by lining shallow pools with rotted manure (horse is best, but poultry manure works too), and then adding the daphnia starter culture after the water turned green. Under these conditions, the daphnia population would quickly “explode”, and the goldfish fry were then added to the pond.  By using several ponds in rotation, the fry could be moved from pond to pond, and would grow quickly on this steady live food source. While most of us do not have the space for several large daphnia ponds, a similar system can be employed using a series of five gallon buckets which can be harvested and refilled every few days. Keep in mind that outdoor cultures that are uncovered will likely be contaminated with mosquito larvae which can lead to a mosquito problem if brought indoors.

a cloud of daphnia feeding on green water
A heavy scoop of daphnia
After adding green water, the daphnia can be seen coming to the surface to feed on algae

Another, perhaps more pleasant, food source for the daphnia is simply green water. I keep my daphnia culture in the garage, in a 30 gallon plastic half barrel with an air stone. The culture is fed daily with a bucket of green water from an outdoor pond. In the winter months, I keep a bright light on one of my goldfish tanks to produce a steady supply of green water.

In about a day’s time, the daphnia have cleared the green water, and every three days or so, I siphon off about a third of the volume from the bottom of the daphnia container. From the siphoned water, I take a 3 inch standard mesh fish net, and swirl it around in the bucket to strain out the mature daphnia which are then fed directly to my fry tanks. I then repeat the straining process with a brine shrimp net for collection of the smaller daphnia, and these can either be fed to the smaller fry, or returned to the main culture tub for grow out.

What I like about feeding daphnia is that they will survive in fresh water, so I can load up my fry tanks and walk away, allowing the fry to safely feed on daphnia for several hours. If I have been traveling, there will always be some readily available daphnia to tide me over until the next batch of bbs have hatched.

Be forewarned that for daphnia to remain in the explosive growth phase, you must keep up the water changes (through harvesting) and green water feeding, or they may crash suddenly, and you will lose your culture. It is a good idea to keep a small backup culture in a jar or bucket for just such occasions. If your culture crashes, you can start over with your backup culture. Starter cultures are often available from fellow hobbyists, local fish club auctions, or the live food section on Aquabid.com.