1. Tank low on water
2. Dirty tank, colored water (how do you even enjoy that?)
3. Big fish in small tank
4. Decorations with no theme, no layout, brightly colored (I'm ok with brightly colored as long as it's done well), and no place for the fish to hide (you know -5 gallon tank with a skull, no fishing sign, and no plants. That kind of thing). Kids tank exempt.
5.No filter (my recent betta came from someone who was told by lfs
that bettas did not like filration or bubblers)
6.Schooling fish by themselves
Really though, I feel bad because I think most people really love their fish, but are just ignorant of a lot of things. And to each his own on the decor