About a week ago, maybe a couple days more, I bought a cory catfish, a black female and a white male molly for my ten gallon tank. Before that I just had a betta, but sadly he died recently. So now I just have a catfish and 2 mollies. Normally, the catfish just chills at the bottom or in his cave, and the mollies just swim around peacefully. But about 2 hours ago the black female started chasing the white male relentlessly. I'm new to having fish and stuff so I'm not sure if its a territorial thing or a mating sort of thing. I think its somewhat odd that the female would be chasing the male though. Any advice?

Fish do some crazy things sometimes. The real time to worry is if the one being chased starts getting his/her fins shredded and/or shows signs of stress. In that case, simple isolation of the aggressor should suffice, maybe with a dose of aquarium salt and API Stress Coat + to help rejuvenate the victim.

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I think in this case she may be wanting to breed. Your tank isn't big enough for babies

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If the female is larger than the male, she'll bully him. I pull mine out and put them in a tank by themselves for a while. After a few times of doing that, they'll usually settle down.