I must say that I agree with this. No criticism of this tank, as such, it's just something I notice of Iwagumi style 'scapes. It's clear that the fish have nowhere to hide and therefore are sort of "forced" into schooling behaviour for comfort or security. The Tet's in my heavily planted tank swim all over the palce and always look very comfortable and relaxed, often playfully ambushing each other from behind the leaf cover...Hi, I'm sorry that my first post here is criticism.
First I have to say, that I really like the look of your tank.
"Thats the way they stay all the time. Must be love "
This is not a "normal" behaviour and this indicates that they do not feel secure. Tetras swim in close groups to diminish risks when they feel threatened. This tank is too small, too "open", it doesn't provide enough plants to feel sheltered. When they feel secure they swim loose. You should provide them a larger, well planted tank to meet their needs.
Greetings from Austria.