Wagtail Swordtail Swordtail
Xiphophorus hellerii
About this strain
The defining trait here is the jet-black fins against a body that can be almost any base color — most commonly red or red-and-white. Every fin, including dorsal, anal, pectoral, and the sword, is solidly pigmented black, creating a bold contrast that reads clearly even across a large tank. The 'wagtail' pattern is a genetic modifier layered over existing body-color strains, so you'll see red wagtail, pineapple wagtail, and so on. When selecting, prioritize specimens where the black fin coloration is dense and unbroken rather than faded or peppered. A staple of the pet trade.
Origin
The wagtail pattern emerged through selective breeding programs, likely in the mid-20th century. It's a well-established commercial trait seen across multiple livebearing genera.