It's not you, it's natural instinct. Quite simply most of your fish are food for larger fish, and birds that dive from above. Any light, bright or not, above a pretty sparsely decorated tank will send your fish hiding, especially as they never know what kingfisher is sitting above the light where they can't see. Dimly lit water is a bit safer to move around in.
In the LFS, usually there are many fish in a small tank, and it's a case of safety in numbers. The fish know there is a much greater chance that the predators will pick off some one else in the school. Also tetras like to be in schools of the same species. They are much more skittish when there is only one or two of their own species about. You can put 20 different species of individual tetra in a tank and they will never school and they will feel lonely. I know people like to see differences in body shape and colour, but many fish like to be in a school of their own type. I don't know how big the tank is, but your tetra would be happier if they were in schools of at least 6 per species.