Chapultepec splitfin

Where they come from

Chapultepec splitfins (Girardinichthys viviparus) are a little livebearing goodeid from central Mexico. The wild story is honestly a bit sad: a lot of their native water has been hammered by pollution, habitat change, and introduced fish. In the hobby they are still around because dedicated keepers keep lines going, and that is a big part of why people bother with them.

Setting up their tank

Think "hard, cool, clean, and roomy" more than "tropical community tank." I have the best luck treating them like a temperate livebearer that appreciates oxygen and elbow room.

• Tank size: a 20 long works for a starter group, but 30-40 gallons is where they start acting natural and you get fewer squabbles.
• Temperature: mid 60s to low 70s F (18-22 C) is a sweet spot. They can handle warmer for a bit, but long stretches hot tend to wear them down.
• Flow and oxygen: they like fresh water and decent surface movement. A sponge filter plus a hang-on-back (or an airstone) is not overkill.
• Hardness and pH: moderately hard to hard water is your friend. Neutral to alkaline is fine. Soft, acidic setups are where I see them slowly fade.
• Layout: dense plants or plant-like cover (hornwort, guppy grass, vallisneria, water sprite) plus open swimming space. Add rocks/wood to break sight lines.
• Substrate: anything works. I like sand or fine gravel so mulm can be siphoned easily.

What to feed them

These fish are not picky once they settle in, but they do best with variety. If you only feed one dry food, they will live, but you will notice slower growth, fewer babies, and more bickering.

• Daily staples: a good quality flake or small pellet, plus something with plant content (spirulina flakes, veggie-based pellets).
• Protein boosters: frozen daphnia, baby brine shrimp, cyclops, mysis, chopped bloodworms (not as the only food).
• Live foods: daphnia and grindal worms are gold for conditioning breeders. Baby brine shrimp is great for fry and for getting picky adults eating.
• Greens: blanched spinach/zucchini, or just let them graze on algae and biofilm if your tank grows it.

How they behave and who they get along with

They are active, always in each other's business, and they sort out a pecking order. Males display a lot and will hassle females if the ratio is tight. They are not "mean" like some cichlids, but they can stress timid fish just by being pushy and fast.

• Best group: 1 male to 2-3 females (or more females). In a bigger tank you can keep multiple males, but expect sparring.
• Good tankmates: other cool-water fish that are not delicate and do not need soft/acid water. Think small robust minnows, some rainbow shiners type setups (where legal/appropriate), or similar hardy species.
• Avoid: slow long-finned fish, tiny shrimp colonies you care about, and most warm-water tropical community mixes. Also avoid fin-nippers or anything that will outcompete them at feeding time.
• Plants: they generally do not shred plants like some livebearers, but they will pick at biofilm and microfauna all day.

Breeding tips

They are livebearers, but not the same as guppies. Goodeids have a different reproductive setup (trophotaenia on the fry), and in my experience they like stability more than constant tinkering. Once a group is settled, they breed pretty reliably.

• Conditioning: keep them cooler (upper 60s F) with frequent water changes and lots of small feedings of daphnia/BBS for a couple weeks.
• Gestation and drops: timing varies, but expect females to carry for several weeks and drop small batches rather than huge clouds like guppies.
• Fry survival: adults will eat fry if they can catch them. Thick floating plants and mossy tangles make a big difference.
• Grow-out: fry grow better with clean water and frequent small foods (BBS, crushed flake, microworms). They can be slower to size up than people expect.

Common problems to watch for

Most issues I have seen with Chapultepec splitfins come from treating them like tropical livebearers and keeping them too warm, too soft, or too dirty. They will hang on for a while, then you start seeing random losses.

• Heat stress: gasping at the surface, hiding, fading color, sudden deaths during warm spells. Add aeration, lower temp, reduce feeding.
• Chronic low-grade water quality: clamped fins, skinny fish that still eat, recurring fungus/fin rot. Bigger water changes and more filtration usually fix it.
• Internal parasites: weight loss despite eating, white/stringy poop. Quarantine new fish and consider targeted treatment if symptoms persist.
• Male harassment: females hiding constantly, ragged fins, jumpy behavior. Add females, add cover, or separate the worst male.
• New tank syndrome with fry: fry drop and then disappear over a few days. Usually it is a combo of not enough micro-food and not enough stable biofilm. Seed the tank, feed small and often, keep water clean.