While mollies are mostly peaceful, they will become very aggressive when crowded, which makes them feel threatened. Talk to your fish store owner about the different species of Mollhyfish and how many you should have in your size aquarium.

Why is my molly fish going crazy?

Signs and symptoms of your fish swimming erratically

There are a number of reasons they could be displaying this behavior, but the most common is parasites. ... Gill or skin flukes, a wormlike parasite, may also cause similar erratic swimming behavior but can be harder to spot.

Do mollies Nip other fish?

More than one molly can be kept in a tank together, however, they have been known to nip the fins of others in their tank. It is recommended that only one male molly be kept in a tank since males will fight with each other to establish a dominant alpha fish.