How do you know when a Molly fish is breeding?

If you have an adult male and an adult female molly, they will breed in almost every case. To make sure you have a pair you need to look at the fin under the fishes belly. The female will have a normal fan shaped fin while the male with have a slender tube shaped fin under his belly. When you see males chasing females with their anal fin bent in the opposite direction, you know they are breeding.

You can tell when they have bred when the female begins to get fatter and fatter as the babies grow inside her. It takes several weeks for the babies to be born.