December 25, 2024, 3:38 pm | Read time: 4 minutes
The bitterling (Rhodeus amarus) is a colorful freshwater fish that stands out not only for its appearance but also for its fascinating reproductive behavior. Anyone looking to keep this lively schooling fish will need a suitable aquarium or pond—and, most importantly, the right mussels.
The bitterling is a unique fish, perfect for aquarium and pond owners seeking something special. With its vibrant coloring and interesting mating behavior, which involves a dependency on mussels, these carp stand out from other species. Once widespread, their populations in the wild have significantly declined due to habitat loss and environmental pressures. Beyond its beauty, the bitterling is also ecologically significant, as it forms a close biological partnership with mussels. What makes this species so special and what conditions are needed to keep it properly in a pond or aquarium?
Origin
The bitterling (Rhodeus amarus) belongs to the carp family and is native to Central Europe, the Ural region, and around the Caspian Sea. Its range extends to France, but it is absent south of the Alps and in Northern Europe.
Its typical habitats are slow-flowing or still waters that are densely planted and have mussel populations. In the 19th century, the bitterling was still widespread in Europe and was among the most common fish in some regions.
The unique reproductive strategy, where mussels serve as a breeding ground, has led to a close dependency on the presence of these mollusks. Due to environmental changes, such as the removal of small bodies of water or river pollution, bitterling populations have drastically declined in many areas. Today, the species is considered rare in Germany and is being reintroduced through targeted measures, including those by NABU (a German nature conservation organization).
Appearance
The bitterling is characterized by a high-backed, laterally compressed body covered with large scales. With an average body size of five to six centimeters, it is one of the smallest carp fish in Europe, although larger specimens can reach up to ten centimeters.
The base color ranges from silvery to olive green, with a distinctive blue-green half-stripe that runs only from the middle of the body to the tail fin. This distinguishes it from bream and other carp, which have a continuous stripe.
The dorsal fin is dark, while the other fins appear pale or slightly transparent. The belly is whitish, sometimes with a pinkish hue. During the spawning season, males display a colorful “wedding dress”: their bellies glow in intense red tones, while their backs and flanks shine silvery.
Behavior
Bitterlings are peaceful and sociable schooling fish, known for their lively behavior. They are diurnal and prefer the company of their own kind. They feel particularly comfortable in groups of eight to sixteen.
Their behavior during the breeding season is particularly striking: males defend their territory and attract females to mussels, which serve as spawning sites. Despite their active nature, bitterlings remain compatible and can be easily kept with other species, provided the water parameters are suitable.
During the spawning season, their behavior is especially fascinating: the courtship dances of the males and their cooperation with the mussels make them a highlight in any aquarium. For their engagement, a varied aquarium design and hiding places are crucial.
Diet
As omnivores, bitterlings are easy to feed. They prefer plant-based food but do not shy away from smaller insects when available. They eat algae, soft plant parts, zooplankton, and insect larvae. Mosquito larvae or worms are also part of their natural diet. In the aquarium, they readily accept flake, live, and frozen food. During breeding, the protein content in their diet should be increased to optimally support the fish.
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Proper Care & Maintenance
A cold-water aquarium with a minimum edge length of 100 centimeters or a garden pond is suitable for keeping bitterlings. The water pH should be between 6.5 and 7.4, with a temperature of 59 to 75 °F (15 to 24 °C). The water hardness should be below 10° dH, meaning it should be relatively soft. Dense planting and a sandy substrate, where mussels can burrow, are essential.
Since bitterlings cannot breed without mussels, there should be one mussel per pair. The pond mussel (Anodonta cygnea) is suitable for this. More than one mussel per bitterling pair could harm them! Additionally, the aquarium or pond should provide enough swimming space, as bitterlings are active swimmers.
Bitterlings kept in ponds do not necessarily need to spend the winter in a tank. However, for overwintering in a garden pond, a depth of at least one meter is required to prevent freezing during severe frost. In the lower water areas, the temperature does not drop below 39 °F (4 °C), which the fish can handle well.
Like many freshwater fish, bitterlings can become ill under poor water conditions or inadequate care. They are particularly sensitive to fluctuations in pH levels or excessive water hardness. Parasites and fungal infections are also potential health risks, making regular water changes and good water quality essential. Otherwise, they are considered easy to care for and can live up to five years.