I have a 30 litre tank with 1 beta, 3 cardinals and 3 rosy's. I have had these fish for around 1 year. All the fish are fine except, one of the rosy tetras is just staying still behind a plant next to the filter of my tank. This has only been occurring for around 3 days. It won't move until I feed them or move stuff in the tank around. The rosy doesn't like to be around the others unlike my cardinal tetras. I do regular water changes, my pH is 7.5 and the temperature of the tank is 27 C. I am unsure if this is normal or something is happening to the rosy.