My fish eat eachother sometimes. One had it's tail fin eaten off. Fish sometimes will fight eachother, and the probably did fight when you where gone. All I can say is either get a bigger tank, or remove some fish.
Hope this helps!
...that produces oxygen? I bought a fish and I would like to keep it in a vase with my bamboo plant for decoration, and I was wondering if there is an easy way to put oxygen in the water, like a common plant I can also stick in the water? PS I don't want to go to the store, is there anything I...