Water fleas - friend or foe?

When something jumps in the aquarium, the diagnosis "water flea" is often made - however, very few animals in our aquariums are really water fleas. Often the little guests are jumping fish or ostracods.

Water fleas in the aquarium

Real water fleas, also called daphnia, are not as common in the aquarium as one might think. An aquarium with a filter is usually much too clean for these small filter feeders and they do not cope well with currents. Here they need much too much energy to row against the current. Their relatively energy-less food does not allow this, so they will starve here despite possibly abundant food. However, in filterless aquariums without water flow, water fleas are useful little helpers, taking care of suspended solids, floating algae and microorganisms floating in the water. They can make an important contribution to clean aquarium water - provided (as already written) that they find enough food in the aquarium and that it is flow-free.

Water fleas in nature

There are several genera of water fleas. The most common water flea in nature is certainly Daphnia pulex, the common water flea, closely followed by Daphnia magna, the large water flea. In addition, however, there are about 90 other species in Europe alone. In nature, water fleas are often found in flocks in stagnant waters, mainly in spring and summer. Daphnia apparently prefer waters that are largely devoid of plants.

Daphnia are extremely sensitive to toxins in the water and are therefore considered to be indicator animals for good water quality.

How can you recognize a water flea?

Daphnia have a typical body structure: they are relatively potbellied, have a detached head, two strong rudder legs, and the body ends in a spine. Water fleas have a large compound eye and a small naulius eye. They sit centrally below each other in the head. In some water flea species the large compound eye is reddish in color, in others it is dark.

Water fleas have a transparent light brown to beige-reddish coloration. In females you can see a so-called saddle on the back, here they form their eggs. It can be light or dark colored, depending on whether the female carries normal eggs or permanent eggs.

Food

Daphnia constantly pump a stream of water through the front of their body, where the pairs of legs are located, with which they on the one hand generate the water flow and with which they on the other hand filter their food from the water. The filter combs are located on the third and fourth pair of legs. They take all particles out of the water that are larger than about 0.5 micrometers. Thus, Daphnia can even filter out bacteria. In addition to bacteria, other microorganisms are eaten, as well as unicellular and multicellular suspended algae, tiny plant debris and other suspended matter that would otherwise cloud the water.

Reproduction

Daphnia can reproduce both sexually and asexually through juvenile production. The asexually produced eggs are light-colored and the saddle of the females looks light-colored. From these eggs hatch the tiny water flea larvae, which are again female when conditions are favorable. If conditions slowly worsen, males also hatch and then mate with the females. This sexual reproduction results in the permanent eggs, which are dark in color. This is especially interesting in the live food breeding of Daphnia: If there is an increased number of female water fleas with dark saddles, this is an indication that the living conditions in the breeding tank tend to become worse. Here you should then search for the cause and take appropriate countermeasures.

Survival strategies

In the fall, the animals die in the field, but their permanent eggs overwinter in the pond mud. The next generation hatches from them the following year if conditions are favorable. Even in adverse conditions such as nutrient deficiency, deterioration of water quality, too cold or too warm temperatures, desiccation and so on, the female water fleas, after mating with the males, form permanent eggs that are very well protected and can survive even drought, frost and heat for several years until the conditions in the biotope are suitable again.

Water fleas as food

Water fleas are an excellent supplementary fish food. They are relatively low in nutrients, but contain a proportionally high amount of chitin, which is an important fiber for fish that they need for healthy intestinal function. However, fish should not be fed purely on water fleas; they are not suitable as a complete food due to their fairly low nutrient density. In fresh substance, daphnia have a water content of 95%, 2.5% protein, 0.5% fat, the rest is fiber. Water fleas are easy to grow as live food in the house, on the balcony or in the garden, but you can also buy them in the frozen food section of the pet shop.

Shrimps also benefit from water fleas. In a filterless aquarium doubly so, of course, because shrimp don't do well with germs in the water. However, these are eaten by the water fleas with preference. Furthermore, as small crustaceans, water fleas must shed their skin regularly, just like shrimp. Shrimps love to eat the shed skins of water fleas - they consist largely of the same basic substances as the shrimp shell: chitin, structural proteins and calcium. This provides the shrimp with these important substances, which they urgently need for the formation of their own exoskeleton. The odd live water flea may also be eaten. In addition to the fiber-rich shell, it also provides protein, moulting hormones and a little fat - for shrimp, which should be fed a diet quite rich in fiber anyway, water fleas available as frozen food are also a highly recommended additional food.

Live food breeding of water fleas

Water fleas are a very easy to breed live food for aquaristics.

All you need is a vessel with water (aquarium water or tap water): an aquarium, a tub, a bucket, ... from 10 to about 60 liters capacity. Aquatic plants do not need to be used. Removal is easier if the animals do not have to be netted out from between aquatic plants. There is also evidence that water fleas feel more comfortable in containers without plants. They are also more likely to be found in nature in water without plants. Beware of filamentous algae formation: If the daphnia get stuck in the algae, it costs them an enormous amount of energy to free themselves again. In the worst case they starve to death. Bottom substrate is also not necessary. However, you can add some brown autumn leaves to the breeding tank. Useful co-inhabitants in the water flea tank are snails: post horn snails, mud snails or bubble snails like to take care of food remains, skinning remains and dead water fleas. Some filter mulch from a running aquarium has also proven helpful: With this you immediately bring in the necessary beneficial microorganisms, which on the one hand make themselves worthy of the water quality and on the other hand serve as food for the daphnia.

Temperature

Daylight on the windowsill is sufficient for lighting. In the open air, of course, you do not need to illuminate at all. Water flea containers in the open air should not get a blazing sun, so that they do not overheat. The ideal water temperature is 20-25 °C. Below this temperature the water fleas reproduce less well, above this temperature oxygen deficiency and overturning of the water can occur, which would result in the death of the daphnia.

Feeding

Feeding is done with slurried dry yeast, spirulina powder, milk (only a little - one drop is enough for a bucket! Too much milk can cause a water flea breeding to tip over unbelievably fast), condensed milk (also only drop by drop), Artemia rearing food such as Liquizell, ... Always feed only so much that the water becomes slightly cloudy. Feed again only when the water is clear again. Optimally, it is better to feed a little more often and a little less. Too much dust food can cause the breeding to topple. It is ideal if an algae bloom spreads in the water flea tank. 

Other

Ideally you should have two to three of these containers in use - on the one hand as a back-up in case a breeding should collapse, on the other hand as a buffer in case the reproduction in one of the breeding containers should collapse.

If there are enough snails in the tank, you do not need to vacuum the substrate - provided the mulm that forms there is loose and flaky brown. Thick, blackish or white, slippery layers that smell bad when removed must be removed immediately. This is where pollutants such as sulfur dioxide or methane are produced that could cause your water flea population to die.

Evaporated water is replenished with aquarium water or, if the water is very hard, half-and-half with aquarium water and water from the demineralizer, distilled water, rainwater filtered with activated carbon, or osmosis water. Water changes are usually not necessary.

Problems with water flea breeding

If the snails in the live food batch no longer lay spawn balls, if you discover many water fleas with black saddles, or if the batch begins to smell unpleasant, you need to act. If the water deteriorates, a generous water change is due. Please siphon off dead animals immediately and do not feed for a while. If food is scarce, carefully increase the amount of food.

Water fleas - where to get them?

You can get the initial stock for your water flea breeding in nature - please note that different rules apply for ponds depending on the federal state, please inform yourself at the responsible authorities (fishery authority, lower nature conservation authority and so on) whether you are allowed to simply do this at your place.
Of course, it is easier and less complicated to get a live batch of water fleas in the pet shop. Make sure that the water in the bag is reasonably clear and that there are not too many dead animals in the package.

Initially, the batch of water fleas must be fed very sparingly so that the water does not tip over. Indirect feeding works optimally here: you actually feed the snails, which then leave behind metabolic products, which in turn feed bacteria again, which are eaten by the water fleas. If you see that the reproduction of the daphnia is taking off, you can switch to direct feeding.