Cordoba livebearer

Where they come from

Cordoba livebearers (Jenynsia obscura) are one-sided livebearers from southern South America. You see them in Argentina, Uruguay, and southern Brazil, cruising clear streams, ditches, and the edges of slow rivers. Cool winters, warm summers, decent hardness, and plenty of oxygen. That seasonal swing explains why they handle cooler water better than most livebearers.

Setting up their tank

Give them room to sprint. A 20-gallon long is the smallest I would use for a small group, and a 29 feels more relaxed. They are quick, surface-oriented swimmers and they jump, so run a tight lid.

• Temperature: 64-74 F (18-23 C). Short dips to 60 F are usually fine. Avoid long spells over 76 F.
• pH and hardness: Neutral to alkaline, moderately hard water (pH 7.0-8.2, 8-20 dGH).
• Flow and oxygen: They like a bit of current and high oxygen. A hang-on-back or canister with strong surface ripple is great. Add an airstone in summer.
• Layout: Sand or fine gravel, open swimming space, plus weedy patches. Vallisneria, hornwort, and floating water sprite work well.
• Cover: Floating plants calm them and give fry places to vanish.
• Lighting: Moderate. They graze biofilm and do fine with some green algae.

Weekly maintenance matters more than gadgets here. Do 30-50% water changes, keep the surface broken for gas exchange, and avoid cranking the heater unless your room is cold.

What to feed them

They are greedy omnivores with a taste for small critters. Think high-protein base with some greens. If you feed only dry food, they get chunky and a bit dull; mixing in live or frozen foods perks them up.

• Staples: Quality flake or 0.5-1 mm micro-pellets with some spirulina.
• Protein: Baby brine shrimp, daphnia, mosquito larvae, blackworms, frozen mysis.
• Greens and roughage: Blanched spinach or zucchini shavings, spirulina flake, green algae they graze.

How they behave and who they get along with

Active, surface to midwater, and a little pushy. Males pester females, and they will sample fins on slow, long-finned fish. Keep them in a group, and aim for a female-heavy ratio to spread out the attention.

• Group advice: 1 male to 2-3 females works. In a bigger tank, 2-3 males with 6-8 females is even better.
• Good neighbors: Fast, cool-tolerant fish like white cloud mountain minnows, peppered corydoras (C. paleatus), hillstream loaches, or hardy, short-finned tetras that like the same temps.
• Use caution: Fancy guppies, bettas, angels, and anything with veils or long fins.
• Inverts: Small shrimp will get picked at. Large, hardy snails are usually fine.

Breeding tips

Fun and quirky part: Jenynsia are one-sided livebearers. Males have a modified anal fin that points either left or right. Females also have a left or right opening. Only compatible orientations produce fry. Since you will not always get both sides in a small group, keep multiple males to improve your odds.

How to spot it: View the male from behind or below. His gonopodium clearly angles left or right. With females, you can see a slight offset at the vent if you look carefully from underneath.

Breeding is otherwise straightforward. Keep them a bit cooler with good food, then bump feeding with live foods. Gestation is roughly 4-6 weeks. Expect 8-20 chunky fry from a mature female. Adults may pick at newborns, so dense floating plants or a well-stuffed spawning mop help. If you want numbers, move the very pregnant female to a planted breeder box and pull her out right after the drop.

Common problems to watch for

• Heat stress: Above mid-70s F they get cranky, breathe fast, and infections show up. Boost surface agitation and cool the room if needed.
• Bullying and fin nips: Crowding, too many males, or slow tankmates invite trouble. Add line-of-sight breaks and rebalance the group.
• Internal parasites on new, wild, or farmed stock: Stringy white poop, weight loss despite eating. Quarantine and treat appropriately.
• Bloat from dry-only diets: Mix in live or frozen food and some roughage. Do not overfeed.
• Jumping: Any spook, and they are on the floor. Use a snug lid and cover gaps.
• Soft or acidic water blues: They look washed out and breed poorly. If your KH is low, add crushed coral or a bit of aragonite to the filter to stabilize pH.