Swordtail Tank Setup: A Real 29 to 30 Gallons and a Lid That Actually Seals

Swordtails need more room than a guppy or molly, sturdier filtration to match, and a lid that seals, since this is a capable jumper, not just a hobbyist exaggeration.

Tank Size

20 gallons is the genuine floor for a small trio, a step up from the 10-gallon tank that gets a small guppy or platy group by. But a group wants 30 gallons or more, and as much length as the space allows. That more generous size is the real target, since swordtails are noticeably more active, open-water swimmers than a molly or platy of similar size. Go long and rectangular rather than tall, and use a lid, more on that below.

A 20-gallon long tank is a workable starting point for a small trio, with a step up to 29 or 30 gallons the better real-world target for most keepers.

Temperature

A tropical range of roughly 65 to 82°F works, with 75 to 79°F the steadier, more comfortable target. Stability matters more than hitting an exact number, so avoid sudden swings. A heater is necessary unless your room stays consistently warm year-round.

Water Chemistry

Swordtails do best with a pH of 7.0 to 8.4 and moderately hard to hard water, roughly 10 to 25 dGH. If your tap water runs soft, a mineral supplement, crushed coral or a commercial buffering product, can bring it into range.

Swordtails do best beside similar-sized, easygoing fish: platies, mollies, cory catfish, rosy barbs, kuhli loaches and otocinclus all fit. Platies are the one caution, since they interbreed with swordtails. Aggressive or boisterous fish are the ones to avoid, because a swordtail does not defend itself and turns timid around them.

The Jumping Risk

This is worth its own section because it's one of the more consequence-heavy details in swordtail keeping, and it's real, not hobbyist exaggeration. Swordtails are powerful jumpers, and a secure lid is the only thing that stops them. Hobbyist reports consistently describe fish escaping through gaps far smaller than you'd expect, a few inches at the back of a lid is often enough.

A full mesh or glass lid with no meaningful gaps is the fix, not an optional upgrade. Leave a feeding hatch or filter cutout no larger than necessary, and check it after any tank rearranging, since even a temporarily propped-open corner is enough.

Filtration

Swordtails need more real filtration capacity than a guppy or molly. Filter flow should run at 4 to 5 times the tank's volume per hour for this species, and a bigger, more active fish in a bigger tank produces a bigger bioload to match. For a standard or growing group, a hang-on-back filter or a stronger canister filter handles the load comfortably.

If fry survival is the priority instead, a dedicated breeding or nursery tank, a gentle sponge filter is the better call, since it won't pull small fry into strong current the way a power filter can.

Substrate

Fine gravel or sand both work well. Avoid sharp or coarse gravel, which can damage fins, especially on longer-finned lyretail and hi-fin varieties.

Lighting

A basic LED light on an 8 to 10 hour daily cycle is enough for the fish and for the plants below, as long as the plants are low-light species. A plug-in timer keeps the day the same length every day.

Planting for Female Refuge

Dense live plants aren't just decoration in a swordtail tank, they're a genuine, practical tool for two separate problems. First, the same problem a molly tank has: swordtail parents don't look after fry, and adults will eat them given the chance, so dense live plants give fry somewhere to hide.

Second, and more specific to this species, planting gives females somewhere to physically break line of sight from a persistent male.

The plants that survive a basic LED are the low-light ones, and hard, alkaline swordtail water narrows the list further. Three cover the job between them:

• Vallisneria 'Tiger' for the back wall. Its striped strap leaves run 30 to 50 cm (12 to 20 inches), it needs no strong light or CO2, and it grows best in hard, carbonate-rich water, drawing its carbon from the carbonates. It spreads by runners into a screen a female can slip behind.
• Hornwort (Ceratophyllum demersum) for fast cover. It is rootless, low-light, low-CO2 and fast-growing, and it can be planted in bunches or left to drift near the surface, where the tangle hides newborn fry.
• Anubias barteri var. nana for the lower levels. It is a slow, low-light plant that grows best tied to rock or driftwood rather than planted in the substrate. Tie it on with fishing line until it grips, and never bury the rhizome, which rots when covered.

Plant the tall species along the back and one end, and leave the front and middle open for swimming. A swordtail is a long open-water fish, and the refuge works when it sits beside the swimming lane, not across it.

The Non-Obvious Requirement: Cycling and Conditioning

Cycle the tank before adding fish. Building the beneficial bacteria colony that processes waste takes 4 to 6 weeks with a fishless cycle, or somewhat less with a bottled bacteria product. Skipping this step is one of the most common beginner mistakes in the hobby generally, and it's a fast route to ammonia-driven stress.

Always use a water conditioner on tap water before it goes in the tank, dechlorinating and neutralizing chloramine and heavy metals that are otherwise harmful to fish. A water test kit lets you confirm the cycle actually finished and catch drift afterward.

The Swordtail encyclopedia profile has more on the species itself.

Sources & Further Reading