Why does my molly flare its fin like a mohawk?

Answer 1

Its either used as an aggressive signal, thus making its self larger or to attract the attention of a female, most fish flare there fins for both purposes.

Answer 2

Male Mollies do that to show off to a potential female mate. Sail fin Mollies will fully extend the dorsal fin while circling the selected female Molly before spawning with her.

Answer 3

it will do this when is is mad or scared, or maybe that's just the way it's fins go. fish have a dorsal fin on their backs that sometime runs up to their heads. when it flares it's fins out all the way, it is trying to make itself look bigger.