Mountain swordtail

Where they come from

Mountain swordtails (Xiphophorus nezahualcoyotl) come from cooler, higher-elevation waters in Mexico. Think clear streams and spring-fed creeks with current, rocks, and a lot of oxygen. That "mountain" background shows up in the aquarium: they usually look best and act most natural in a tank that is not run like a warm, slow guppy puddle.

Setting up their tank

Give them room to move. They are active, always cruising, and males will posture and chase. I would not keep them in a tiny tank just because they are livebearers.

• Tank size: 30 gallons for a small group is a comfortable starting point. Bigger is nicer if you want multiple males.
• Filtration: a filter that moves water (HOB or canister) plus extra aeration if the tank is warm or stocked.
• Flow: moderate current is appreciated. You do not need a river tank, but dead-still water is not their vibe.
• Hardscape: rocks, rounded stones, and driftwood give sight breaks so the weaker fish can get away.
• Plants: sturdy plants (Java fern, Anubias, vallisneria) and floating plants help calm the tank and provide fry cover. Keep some open swimming space.
• Substrate: anything is fine. I like sand or fine gravel with a few rock piles to mimic their home waters.

For water, I have had the best luck in slightly cooler freshwater with good oxygen. Room temp works in a lot of homes. If your tank runs hot year-round, add flow and surface agitation and watch for fish hanging near the top.

What to feed them

These are easy eaters, but they do better when you feed like an omnivore, not just "flakes forever." In the wild they pick at algae, biofilm, tiny insects, and whatever drifts by. In the tank, variety keeps them in good weight and helps females drop healthier fry.

• Staple: a quality flake or small pellet that includes some plant content (spirulina/veg).
• Greens: blanched zucchini, spinach, or a tiny piece of nori clipped to the glass.
• Protein: frozen or live foods a few times a week (daphnia, brine shrimp, mysis, bloodworms in moderation).
• Fry food: crushed flake, baby brine shrimp, microworms, or a fine fry crumble.

How they behave and who they get along with

They are busy, social fish. Males can be pushy with each other and they will pester females if the ratio is off. You will see sparring and showing off - that is normal - but constant cornering is a sign you need more space, more cover, or a better male-to-female balance.

• Group size: keep them in groups, not singles. They are less stressed with their own kind around.
• Sex ratio: aim for 1 male to 2-3 females. If you keep multiple males, the tank needs extra room and lots of line-of-sight breaks.
• Good tankmates: other peaceful fish that like similar temps and clean water (many rainbowfish, danios in bigger tanks, Corydoras, bristlenose pleco).
• Use caution with: long-finned slow fish (they may nip), tiny shrimp (fry and small shrimp can be snacks), and aggressive fin-nippers (they will get stressed).

Breeding tips

Like other swordtails, they are livebearers, so breeding is not hard. The trick is managing the social pressure and what you want to do with the fry. If you keep males and females together, you will get babies sooner or later.

• Conditioning: feed a bit heavier with quality foods for 1-2 weeks, then do a big water change. That combo often triggers drops.
• Birthing cover: dense plants (especially floating plants) give fry a chance. Bare tanks produce fewer survivors because adults will snack.
• Fry rearing: if you want numbers, move a pregnant female to a separate tank before she drops, then move her back out right after.
• Culling/selection: watch for weak spines or poor swimming. Keeping only the strong ones keeps the line healthier over time.

Common problems to watch for

Most issues I see with mountain swordtails come from warm, low-oxygen tanks, crowding, or nonstop male harassment. Fix the environment and you fix a lot of "mystery" problems.

• Shimmying or hanging at the surface: often tied to stress, low oxygen, or unstable water. Add aeration, check ammonia/nitrite, and do a water change.
• Nipped fins and worn tails: usually too many males, not enough cover, or incompatible tankmates. Add sight breaks and reconsider the stocking.
• Ich and other parasites: can show up after new fish or big temp swings. Quarantine new arrivals if you can.
• Bloat/stringy poop: can be overfeeding, too much rich food, or internal parasites. Fast a day, feed daphnia/veg, and observe. Treat if it does not improve.
• Females getting skinny after drops: they may be getting outcompeted at feeding time. Target feed and give them places to hide from males.