My sister has a 30 gallon freshwater tank at her apartment. In it she has 4 Zebra Danios. I'm not sure why she picked 4 but anyway... those are the only fish in her tank. Is it normal for these fish to be aggressive? Out of the 4 one imparticular seems to be quite a bully. It chases the other fish around the tank and apparantly nips at them. When my sister told me, I had to see for myself. Sure enough, this one fish is kinda nasty to the others. Maybe we are misinterpreting what is going on, but we are both concerned. We don't want any of the other 3 to get hurt. Out of the 4, the bully is only slightly larger and by this I mean maybe a millimeter if at all. Does this matter? What is going on here?