Quick Stats
| Adult Size | 5–7 cm |
| Minimum Tank | 20 gal |
| Temperature | 10–22°C (unheated) |
| pH Range | 6.5–8.0 |
| Hardness (GH) | 5–15 dGH |
| Difficulty | Medium |
| Temperament | Peaceful (males territorial during breeding) |
| Diet | Omnivore — flakes, micro pellets, frozen daphnia, brine shrimp, blanched vegetables |
| Schooling | 6+ recommended |
Tank Setup
Bitterling (Rhodeus amarus) are a small European cyprinid native to slow-moving rivers and ponds. They tolerate temperatures from 10–22°C, making them a perfect unheated-tank fish for a cool house. A 20 gallon tank is the minimum for a group of 6–8.
Water parameters: temperature 10–22°C (unheated in a cool house or basement is ideal), pH 6.5–8.0, hardness 5–15 dGH. They prefer still or slow-moving water — unlike hillstream species, they don't need strong current. Standard filtration is fine.
Set up the tank with a sandy substrate, driftwood, hardy plants (Vallisneria, Java fern, Anubias), and plenty of swimming space. If you want to breed them, you MUST provide live freshwater mussels (Swan Mussels, Anodonta cygnea) — the bitterling's entire breeding strategy depends on them. Without mussels, no breeding occurs.
Tank Mates
Bitterling are peaceful outside of breeding season. Males become territorial during spawning (displaying to rivals over mussels), but aggression is mild and limited to chasing. They work in any cold-water community where the temperature stays below 22°C.
Compatible tank mates: White Cloud Mountain Minnows (perfect cold-water overlap), Vietnamese Cardinal Minnows, Rosy Barbs (at the cool end), Weather Loaches, Cherry Shrimp, Amano Shrimp, and Japanese Trapdoor Snails. They coexist with goldfish in a large tank, though goldfish may outcompete them for food.
Avoid: tropical species that need 24°C+ (bitterling will slowly decline), aggressive species (cichlids, large catfish), and fish large enough to eat a 6 cm bitterling. Also avoid housing with species that dig up mussels (goldfish will root through the substrate and damage mussel beds).
Diet & Feeding
Bitterling are omnivores. They accept flakes, micro pellets, frozen daphnia, frozen brine shrimp, frozen bloodworms, and blanched vegetables (spinach, zucchini). In a mature tank they graze on micro-organisms and soft algae.
Feed once or twice daily. They are surface and mid-water feeders, so floating or slow-sinking foods work best. A varied diet is essential for breeding condition — males only develop their full breeding colours (iridescent blue-green on the sides, orange-red in the fins) when in top condition.
Condition breeders with live or frozen food daily for 2 weeks before introducing mussels. The standard flake-only diet keeps them alive but fades their colours and suppresses breeding behaviour.
Common Health Issues
Bitterling are hardy when kept cool. The main health issue is heat stress — temperatures above 24°C suppress breeding, weaken the immune system, and shorten lifespan. An unheated tank in a cool house is genuinely better for them than a heated tropical tank.
They are susceptible to ich when stressed by temperature swings. The standard heat-and-salt treatment works, but keep the treatment temperature under 24°C. They are sensitive to copper — many fish medications contain it; always check the label.
Lifespan is typically 5–7 years. They are longer-lived than most small cyprinids (White Clouds, danios) when kept in cool water. The species is declining in the wild due to habitat loss and the decline of freshwater mussel populations — captive breeding matters for conservation.
Breeding
Bitterling have one of the most extraordinary breeding strategies in the animal kingdom. Females develop a long, tubular ovipositor (up to 5 cm long — longer than the fish itself) that they insert into the excurrent siphon of a living freshwater mussel. The eggs are deposited inside the mussel's gill chamber, where they develop safely for 3–4 weeks before the fry emerge.
Males defend territories around mussels and display to females. Spawning happens in spring, triggered by a temperature rise to 18–20°C. To breed deliberately, condition a group with high-protein frozen foods for 2 weeks, then add 1–2 live freshwater mussels (Swan Mussels, Anodonta cygnea) per male.
Fry emerge from the mussel at about 1 cm long and are immediately free-swimming. Feed them infusoria for the first week, then microworms and baby brine shrimp. Without mussels, no breeding occurs — this is the limiting factor for home breeding. Swan Mussels themselves need mature, well-filtered water and are not easy to keep alive long-term.
Frequently Asked Questions
Do Bitterling really lay eggs inside mussels?
Yes — it's one of the most extraordinary breeding strategies in nature. Females develop a long tubular ovipositor that they insert into a living freshwater mussel's excurrent siphon. The eggs develop inside the mussel's gill chamber for 3–4 weeks before fry emerge. The mussel gets nothing from the arrangement — bitterling are parasitic breeders.
Do Bitterling need a heater?
No — they are European cold-water fish that prefer 10–22°C. An unheated tank in a cool house or basement is ideal. Sustained temperatures above 24°C suppress breeding and shorten their lifespan. They tolerate brief freezes but should not be kept below 5°C long-term.
Where can I buy Bitterling?
They are not commonly carried by big-box pet stores. Look for them at North American native-fish suppliers (e.g., Zimmerman's, Jonah's Aquarium), European aquarium societies (the species is native to Europe and more available there), or online auction sites. Captive-bred stock is rarer than wild-caught.
Can I breed Bitterling without mussels?
No — the entire breeding strategy depends on freshwater mussels. Females cannot deposit eggs without a mussel's excurrent siphon. To breed bitterling, you need 1–2 live Swan Mussels (Anodonta cygnea) per male. The mussels themselves need mature, well-filtered water and are not easy to keep alive long-term.