Red swordtails are crossbreeds, developed by mixing green swordtails with red platies. Several different varieties of red swordtails exist, including velvet red, brick-red and red wag swordtails. Since all these varieties are closely related and share the same water requirements, they can be housed with the same kinds of tank mates.
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As social fish, red swordtails need to be kept with others of their own species. Those who are housed alone will become shy and withdrawn. Males of the species can become territorial and act aggressively toward one another. Unless you have an exceptionally large tank, it's best to keep just one male along with two or more females. You can easily tell the sexes apart, as males have a long, sword-like extension from their tail fins.
Peaceful catfish species make ideal tank-mates for red swordtails. Both corydoras and plecostomus are good choices. These varieties are peaceful toward other fish and share similar water requirements to red swordtails. Plecostomus require large tanks -- 75 gallons or more -- as they can grow up to 18 inches long, but can be kept alone. Corydoras are smaller, often around 3 inches long, but been to be kept in groups of at least six members.
Some species of tetra can live with red swordtails. Since swordtails tend to measure between 4 and 4 1/2 inches long, larger tetras are better suited so they don't end up getting bullied. Suitable species include bleeding heart tetras, Congo tetras, emperor tetras and red eye tetras. If you're unsure whether a particular species is compatible, check with a specialist aquarium supply store or fish breeder. All tetras are schooling fish, so they need to be kept in groups of six or more of their own species.
Since platies belong to the same taxonomic family as red swordtails, they share extremely similar care requirements and can be ideal tank-mates. Other potential companions include mollies, angelfish and most other peaceful fish of a similar size or slightly smaller. Avoid any species that are known fin-nippers as they may bully your red swordtails and go after their flowing fins.