Several fish can have a black mark near the tail, so the mark alone cannot identify the species. A sailfin molly may have a rounded tail tipped in black; a tail-spot wrasse has a distinct blue-edged black spot at the base of its tail. First check exactly where the mark sits, then compare the fish’s habitat, body shape and other markings.
First check where the mark is
- On the tail fin’s edge: A black tip on a rounded tail can fit a sailfin molly.
- At the base of the tail: A distinct spot, especially one edged in blue, can fit a tail-spot wrasse.
- On the belly: A dark mark near the back of a female livebearer’s abdomen may be a gravid spot, not a tail marking. Aqueon notes that this spot can be hard or impossible to see in darker-colored livebearers.
Fish that may match
Sailfin molly
The Florida Museum of Natural History describes the sailfin molly (Poecilia latipinna) as having a broad area where the tail joins the body and a large, rounded tail fin that is sometimes tipped with black. Aquarium-bred mollies come in many color variations, so color alone cannot confirm the identification. Florida Museum of Natural History’s sailfin molly profile gives the identifying traits.
Tail-spot wrasse
The tail-spot wrasse (Halichoeres melanurus) has a blue-edged black spot at the base of the tail and a second spot in the middle of its dorsal fin, according to the Oregon Coast Aquarium’s species profile. This is a stronger possibility if the fish is marine and has both spots. The aquarium reports a maximum length of about 12 cm and notes that the species begins life as female and may change sex to male.
Livebearer with a gravid spot
Mollies, platies, swordtails and guppies are among the familiar livebearers. In females, a gravid spot is on the abdomen toward the back of the belly, not on the tail. Aqueon’s livebearer care guide explains the spot’s position; its livebearer aquarium setup guide notes that it may be difficult to see on darker varieties.
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How to narrow down the identification
- Pinpoint the mark. Look at a clear side view and distinguish the tail fin itself from the base of the tail and the belly.
- Check the setting. Note whether the fish lives in a freshwater aquarium, a marine aquarium or a natural waterway. A molly is a freshwater livebearer example; the tail-spot wrasse is a marine example.
- Look for other features. A second spot on the dorsal fin supports the tail-spot wrasse possibility. Body form and fin shape can help assess a possible molly.
- Consider variation. Livebearers have many bred color and fin forms, so a color pattern by itself may not establish the species.
A clear side-view photo, the fish’s freshwater or saltwater setting, and the spot’s precise location are the most useful details for narrowing it down. Without those clues, the fish cannot be identified reliably from “a black spot on its tail” alone.
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