Onesided livebearer
Jenynsia lineata
Onesided livebearers feature a slender body with a distinctive lateral stripe and a mix of iridescent blue and green coloration.
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About the Onesided livebearer
A quirky South American livebearer that hugs the mid-to-top of the tank and shows off broken stripe patterns. Females get much bigger than the little males, and their odd one-sided mating gear is a fun bit of fish trivia to share. Give them cooler, clean water and a small group and they settle in nicely.
Also known as
Quick Facts
Size
12 cm
Temperament
Semi-aggressive
Difficulty
Intermediate
Min Tank Size
29 gallons
Lifespan
2-3 years
Origin
South America
Diet
Omnivore - quality flakes/pellets, plus live or frozen foods and some greens/algae
Water Parameters
18-25°C
7-8
10-20 dGH
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This species needs 18-25°C in a 29 gallon tank. Use our heater calculator to find the right wattage.
Calculate heater sizeCare Notes
- Give a group of 6+ a 20-gallon long with open swim space, some rocks/roots, and moderate flow; they jump, so run a tight lid.
- Target pH 7.2-8.2, GH 10-20, KH 6-12 at 64-75 F. They sulk in soft water, so add crushed coral or aragonite; a pinch of salt (1-2 g/L) is optional, not required.
- Feed small portions twice daily of quality flake or 0.5-1 mm pellets, plus live/frozen brine shrimp, daphnia, mosquito larvae, and a bit of spirulina or blanched veg; they pack on weight if you overfeed.
- Pair them with quick, cool-water fish like zebra danios, white clouds, and sturdy livebearers; skip bettas, fancy guppies, long-finned tetras, and most shrimp.
- Males are pushy, so run 2-3 females per male and add sight breaks (plants, rock piles) so the girls can duck away.
- They are true onesided livebearers: lefty males only mate with righty females (and vice versa), so keep both sides if you want fry. Gestation is about 4-6 weeks and adults will hunt babies, so use dense floaters or move the mom.
- Heat and low oxygen knock them out fast; keep strong surface agitation and avoid temps over 77 F. Quarantine new fish and watch for camallanus (red threads from the vent) if they are wild-caught.
Compatibility
Good Tankmates
- Fast, cool-tolerant schooling fish like zebra or pearl danios that can shrug off a bit of chasing
- White cloud mountain minnows (standard form) - quick enough to avoid nips and happy in the same cooler, harder water
- Robust, active tetras that handle cooler water, like bloodfins or Buenos Aires tetras
- Peppered corydoras and other hardy Corys that stick to the bottom; onesideds mostly ignore them
- Bristlenose plecos or other armored algae grazers; too tough to bother and they mind their own business
- Sturdy livebearers like platies (and swordtails if kept on the cooler side) so any sparring gets spread around
Avoid
- Slow fish with fancy fins like bettas, fancy guppies, or angelfish - they will get their fins chewed
- Delicate soft-water nanos like neon or ember tetras and most rasboras - wrong water and they get harassed
- Dwarf shrimp colonies - adults might scrape by, but shrimplets become snacks
- Other notorious fin-nippers or bullies like tiger barbs or serpae tetras - just turns into a bitey mess
Where they come from
Onesided livebearers (Jenynsia lineata) are from the southern end of South America - southern Brazil, Uruguay, and northern Argentina. Think weedy ditches, slow streams, and ponds that cool off in winter and warm up in summer. That seasonal swing is why they do well in unheated rooms and appreciate cooler water compared to a lot of tropical livebearers.
Setting up their tank
They like hard, alkaline water with good oxygen and some current. A 20-gallon long is a nice starting point for a small group. Give them space to cruise and pockets of cover so the girls can get a break from the boys.
- Tank size: 20 gallons or larger for a group
- Temperature: 64-75 F (18-24 C). Short dips to 60 F (15-16 C) are usually fine. Avoid keeping them at 78-80 F long term.
- pH: 7.2-8.2
- Hardness: medium-hard to hard (8-20 dGH)
- Flow and oxygen: moderate filter flow, lots of surface agitation
- Substrate and decor: sand or fine gravel, rocks, and hardy plants (vallisneria, hornwort, water sprite). Floating plants help the shy ones feel safe.
Use a tight lid. They are quick and will find gaps.
They handle a touch of salt, but it is not required. If you keep plants, skip routine salting. Clean water and mineral content matter more.
What to feed them
Omnivores with a real taste for tiny critters and a bit of green. They spend a lot of time picking at surfaces, so give them variety.
- Staples: quality livebearer flakes or small pellets, spirulina flakes
- Meaty foods: baby brine shrimp, daphnia, cyclops, finely chopped bloodworms (as a treat)
- Greens and roughage: blanched spinach/zucchini slivers, algae wafers, and let some algae grow on rocks
- Feeding rhythm: small portions 2-3 times a day so they do not gorge
If you see stringy, pale poop or they look a bit bloated, swap a couple meals for daphnia or spirulina-heavy foods. It clears them up fast.
How they behave and who they get along with
Active, curious, and a little pushy. Males court hard and can pester females, so keep them in a group with more females than males. They will nip slow, long fins if cramped or bored.
- Good tankmates (cool to temperate water): white cloud mountain minnows, tanichthys variants, danios, hillstream loaches, small Corydoras from cooler ranges, goodeids of similar size.
- Use caution: fancy guppies, bettas, long-finned tetras - easy targets for nips.
- Avoid: large cichlids or anything that will view them as snacks.
They do best in a group. Aim for 6+ with a 1:2 or 1:3 male-to-female ratio to spread attention.
Breeding tips
They are livebearers, but with a twist. Males are either left-handed or right-handed - their anal fin (gonopodium) bends to one side. Females are sided too. Mating only works when a male's orientation matches a female's orientation.
Check sides: look from behind the male. If his gonopodium points to his left, he is a lefty. Females have the genital opening slightly offset; you can usually spot it with patience and good light. Keep both left and right types in your group for consistent fry.
Gestation is roughly 4-6 weeks. Females often release fry over several days rather than one big drop. Adults will pick off some babies, so give the fry cover.
- Provide dense plants or spawning mops so fry can hide.
- Temperature around 70 F (21 C) and frequent small water changes keep them cycling.
- First foods: baby brine shrimp, microworms, crushed flake. They are sturdy and grow well on BBS.
- Females can store sperm, so you may get several broods even if a male is removed.
Common problems to watch for
- Too-warm water: they get sluggish, nip more, and live shorter lives. Keep them in the 60s-70s F, not high 70s to 80s.
- Soft, acidic water: leads to shimmying, poor appetite, and thin bodies. Add mineral content or use harder source water.
- Male harassment: torn fins and stressed females. Fix by adding cover, upping tank size, or shifting the sex ratio.
- Fin nipping: usually from crowding or boredom. Increase space and flow, add sight breaks, and keep them with compatible fish.
- Breeding fails: often just a mismatch of left/right orientations. Make sure you actually have both sides represented.
Quarantine new fish. Wild-caught or farmed ones can bring in external parasites. A few weeks in a separate tank lets you treat gently and observe before mixing with your main group.
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