Southern mountain swordtail

Where they come from

Southern mountain swordtails (Xiphophorus monticolus) are from southern Mexico, living up in cooler hill-country streams. Think clear water, steady current, lots of rocks, leaf litter, and changing seasons. They are not your typical warm, hard-water pet store swordtail, and thats where most people get tripped up.

Setting up their tank

Start with more tank than you think you need. They are active, and the males will hassle each other in tight spaces. I would not do them in a nano, even if you see them sold small.

• Tank size: 20 long is a bare minimum for a small group, but 30-40 gallons gives you way fewer headaches
• Filtration: strong filtration plus real surface agitation (they like oxygen-rich water)
• Flow: a powerhead or a filter outlet aimed down the length of the tank makes them act way more natural
• Scape: rocks, rounded stones, wood, and clumps of plants to break lines of sight
• Plants: tough stuff like Java fern, Anubias, Vallisneria, and floating plants for fry cover
• Substrate: sand or small gravel, nothing fancy

Temperature is the big one. Aim cooler than most tropical community tanks. Mid to upper 60s F to low 70s F is where Ive had the best luck long-term. They can handle warmer spells, but they dont love being cooked at 78-80F all year.

Water chemistry: mine did fine around neutral to slightly alkaline. Stability matters more than chasing numbers. If your water is very soft and acidic, you may need to add some mineral back (or mix in harder water) so they arent living in a mineral deficit.

What to feed them

They are easy to feed, but they look their best with variety. In the wild they pick at algae, tiny bugs, and whatever the current brings, so give them a mix of plant-y foods and protein.

• Staple: a good quality flake or small pellet that isnt all fishmeal
• Greens: spirulina flake, blanched zucchini, or a little Repashy-style gel food if you use it
• Protein: frozen daphnia, cyclops, brine shrimp, and the occasional bloodworm (not as the daily driver)
• Fry: crushed flake, baby brine shrimp, microworms

How they behave and who they get along with

They are busy, constantly cruising, and the males do the usual swordtail routine: posturing, sparring, and chasing. It looks dramatic, but its mostly bluffing if the tank has space and cover.

• Group size: keep them in groups, not pairs. More females than males helps a lot (2-3 females per male is a good target).
• Tankmates: think cool-water, non-nippy fish that like current - white cloud mountain minnows, some danios, small peaceful hillstream-type species, and similar.
• Avoid: fin nippers (some barbs, aggressive tetras), very warm-water fish, and big bullies that will outcompete them at feeding time.

Breeding tips

If your adults are settled, they will breed. The trick is raising fry without turning the tank into a nonstop survival game. Adults will snack on babies when they notice them, and tankmates will too.

• Give fry a chance: floating plants, moss, and dense plant clumps are your best friend
• Use a separate fry grow-out if you want numbers (a simple 10-20 gallon works)
• Cooler water tends to mean slower growth, so dont panic if they arent big in a few weeks
• Frequent small feedings for fry beat one big dump of food

Common problems to watch for

Most issues I see with monticolus come from treating them like standard tropical livebearers: too warm, not enough oxygen, and too much food in a small tank.

• Heat stress: hiding, heavy breathing, repeated outbreaks of disease - fix temp and boost surface agitation
• Low oxygen: hanging at the surface, rapid gill movement - increase flow and aeration, clean clogged media
• Bloat/constipation: swollen belly, stringy poop - cut rich foods, add greens, try daphnia
• Shimmies and weakness: often tied to mineral-poor water or unstable parameters - stabilize and consider adding minerals
• Fin damage from sparring: usually a space/cover problem - add sight breaks, reduce male count, or upgrade tank