San Marcos redtail splitfin

1) Where they come from (the quick, interesting version)

San Marcos redtail splitfins (Xenotoca doadrioi) are one of those goodeids that make you feel like you’re keeping something special. They come from central Mexico and, like a lot of goodeids, their wild populations have had a rough time because their habitats got altered and polluted. In the hobby, we’re basically the backup plan—so keeping them well (and breeding them) actually matters.

2) Setting up their tank

Think “active livebearer with opinions.” They’re always on the move, always grazing, and they appreciate room more than fancy decor. I’ve had the best luck giving them a longer tank footprint and keeping things stable rather than chasing exact numbers.

• Tank size: I’d start at 20 long for a small group; 30–40 breeder is where they really look relaxed and social.
• Group size: 6+ is nice. They spread out the bossiness and you see better behavior.
• Filtration: moderate to strong is fine—these fish don’t mind flow. Just don’t blast them with a powerhead in a tiny tank.
• Temp: cool to mid-range works best (roughly 68–74°F / 20–23°C). They can handle warmer short-term, but they get more short-tempered and burn through food faster.
• Hardness/pH: they generally prefer harder, more alkaline water. If your tap is liquid rock, you’re in luck.
• Decor: plants + open swimming space. Rocks/wood to break line-of-sight helps a ton with male squabbles.

Plants are a mixed bag. They won’t bulldoze a scape like goldfish, but they do peck all day. Tough stuff (Java fern, Anubias, moss, guppy grass, hornwort) holds up better than delicate stems unless you’re okay with them looking “lived in.”

3) What to feed them

These guys eat like they’re perpetually offended the buffet isn’t bigger. They’re not picky, but they do better when you lean into their natural “constant picker” style—more veg and small foods, less big heavy meals.

• Staple: a good quality flake or small pellet made for livebearers/omnivores
• Veg rotation: spirulina flake, blanched zucchini, spinach, peas (shelled), algae wafers broken up
• Protein treats: daphnia, brine shrimp, cyclops, blackworms (go easy), frozen mixes
• For fry: baby brine shrimp, crushed flake, microworms—little and often

4) Behavior and tankmates

They’re social, curious, and a bit pushy. Males posture and spar, and they’ll chase each other around like they’re auditioning for a soap opera. In a roomy tank with a good ratio, it’s entertaining. In a cramped tank, it turns into stress.

• Male/female ratio: aim for more females than males (something like 1 male to 2–3 females) to keep the pressure down
• They’ll nip slow, flowy-finned fish (think fancy guppies, bettas, long-fin anything)
• Best tankmates are sturdy, similar-speed fish that like cooler, harder water
• They also do well in a species tank, which is honestly where they shine

I’ve found they settle down a lot when they have something to do—algae to graze, plants to pick through, and enough room to establish a pecking order without trapping the lowest fish in a corner.

5) Breeding tips (they’re goodeids, so it’s a little different)

Breeding is absolutely doable at home, and it’s one of the most rewarding parts of keeping them. Goodeids are livebearers, but not like guppies—females tend to have fewer, larger fry, and the babies are pretty capable from day one.

• Conditioning: lots of greens + small protein foods, and keep the water clean and steady
• Cover: dense plants/moss increases fry survival (adults will snack if they find them)
• Fry setup: a separate grow-out tank makes life easier if you want numbers
• Feeding fry: baby brine shrimp is the growth cheat code; mix in crushed spirulina flake early

If you’re trying to actually preserve a line (not just make more fish), keep them in a dedicated tank and avoid mixing with other Xenotoca/goodeids. It’s way too easy to end up with mystery hybrids once you’ve got multiple species going.

6) Common problems to watch for

Most issues I’ve seen with this species come from three things: too warm, too cramped, or too much rich food. Fix those, and they’re usually pretty hardy.

• Chasing/fin damage: usually a space or ratio problem—add cover, increase tank size, or reduce extra males
• Bloat/stringy poop: cut back on heavy frozen foods, add more veg/spirulina, and keep feeding smaller portions
• Sudden losses after a move: they can be touchy about swings—match temperature and parameters during acclimation, and keep nitrates down with regular water changes
• Ich/stress spots: often shows up after temperature spikes or bullying; treat the cause along with the symptoms
• New fish fading/hiding: give them time and lots of cover—new groups can be skittish until the social order settles