@Philipine eagle Now that you mention it, it is a crowded tank - cutting the school in half or a little less than half would be in keeping with the tank. You're right about them being very active. I currently have six Congo Tetras that are all about 3 years old (fully grown) in a 55 gallon tank and even that seems small for them sometimes, in part because there are eight other similarly sized fish in the tank with them and a lot of plants.
They are fish that like to swim in a larger group, so I think the group size is ok, but the size of the tank is not. Mine was 350 liter / 77 gallon with 120x50 cm dimensions and I was always under the impression they could use more spacing as they liked to race from one side to the other. I kept 16 of them, at that group size they show a lot of colour and interaction. This was definitely one of my favourite species.
