it means there are too many fish for the tank to comfortable support. This means the filter can't handle their waste, there is not enough room for them to swim comfortable. Aggression will become ore apparent.
The old rule of 1 inch of fish per gallon is a falsehood. Even neon tetras, some of the smallest fish around, need room to swim and school.
Also when setting up a tank,remember thatt adding gravel, plants and decorations cuts down onavailablee space for them to swim and live. so a ten gallon might only really be a eight gallon tank. It's always best to research your wanted fish before buying. Most articles on fish tell you the best size tank to keep them in.
