Vietnamese bitterling

Where they come from

Vietnamese bitterlings (Acheilognathus fasciodorsalis) come from freshwater in Vietnam, living in slower stretches of rivers and connected backwaters where there are plants, soft bottoms, and - this part matters a lot - freshwater mussels. They are small, pretty fish, but they are not a "toss them in any community tank" kind of cyprinid.

Setting up their tank

Think "cooler, clean, oxygen-rich" more than "warm tropical." I have the best luck keeping bitterlings in a tank that feels like a calm stream edge: good filtration, decent flow, and plenty of plants or structure so they are not constantly in each other's faces.

• Tank size: 20-30 gallons minimum for a small group; bigger is noticeably easier for aggression and breeding behavior
• Group size: 6+ if you can (they settle in better and you see nicer behavior)
• Substrate: sand or fine gravel; they pick around and it looks natural
• Hardscape: smooth stones, small branches, and plant thickets to break sight lines
• Plants: hardy stems and floaters work well; bitterlings like cover and calm corners
• Filtration: strong biological filtration; they hate old, sour water
• Temperature: mid 60s to low 70s F is a good target (they can handle warmer, but they get more stressed and snippy)
• pH and hardness: neutral-ish is fine; stable beats chasing numbers

If you want to breed them, you will eventually be thinking about mussels. More on that below. Even if you are not breeding, I still set the tank up as if I might: mature biofilter, stable parameters, and no big swings from missed water changes.

What to feed them

They are not fussy once settled, but they do better with variety. Mine colored up and held condition best on small foods they can pick at all day, with a quality staple to keep things consistent.

• Staple: small sinking pellets or fine granules (they like to forage lower in the water)
• Frozen: daphnia, cyclops, baby brine shrimp, bloodworms (use bloodworms as a treat, not every day)
• Live (if you can): daphnia and baby brine shrimp are gold for conditioning
• Occasional: crushed flakes if that is what you have, but do not rely on it

How they behave and who they get along with

They are active, curious, and a bit opinionated. Males can get territorial, especially in breeding condition, and they will spar with each other. Most of the time it is posturing and short chases, but in tight quarters it can turn into constant harassment.

Tankmates should be calm, not too boisterous, and ideally comfortable in cooler water. Fast nippy fish stress them out, and big fish can keep them pinned in hiding.

• Good fits: other small, peaceful coolwater cyprinids, hillstream-type fish that like clean water (if the flow suits everyone), and calm bottom dwellers
• Avoid: fin nippers, hyperactive barbs, aggressive fish, and anything that bullies them at feeding time
• Also avoid: species that will constantly investigate or harass mussels if you are trying to breed

Breeding tips

Bitterlings are famous for their weird, awesome breeding strategy: they lay eggs in live freshwater mussels, and the embryos develop inside. That means breeding is advanced for two reasons: you have to get the fish into condition, and you have to keep the mussels alive and healthy.

If you can source captive-held or legally obtained freshwater mussels that are known to do well in aquaria, keep them in a mature, stable tank with fine sand and good oxygen. Mussels are filter feeders and they do not like dirty, stagnant tanks, but they also do not like being blasted by a powerhead.

• Conditioning: cooler clean water, heavy feeding of small live/frozen foods, and lots of water changes
• Spawning setup: plants and cover plus a few mussels partially buried in sand in a calmer area
• Behavior: males display and guard; females develop a longer ovipositor when ready
• Patience: sometimes they need a seasonal cue (slightly cooler period, then gradual warm-up) to really get going

Raising fry can be tricky because the early stages are tied to the mussel. If you succeed, you will eventually see tiny youngsters appear in the tank. At that point, gentle filtration (sponge is your friend), lots of microfoods, and not getting outcompeted by adults becomes the main challenge.

Common problems to watch for

• New import stress: hiding, clamped fins, not eating - give them time, low traffic, and pristine water
• Old water syndrome: they can look fine, then suddenly get lethargic or start flashing - regular water changes help a lot
• Aggression in cramped tanks: frayed fins and constant chasing - add space, cover, or reduce male-to-male pressure
• Heat and low oxygen: hanging at the surface or breathing hard - add surface agitation and consider cooling in summer
• Parasites (common in wild fish): weight loss despite eating, stringy poop - quarantine and treat based on what you actually observe

The biggest "secret" with this species is consistency. Once you give them clean, steady water, a little cool-down from typical tropical temps, and enough room to spread out, they stop acting fragile and start acting like the confident little river fish they are.