Why are the rummy nose tetras lying on the bottom of the tank?

Rummy nose tetras lying on the bottom of the tank could be experiencing stress due to poor water quality, inadequate tank conditions, or illness. They may also be seeking refuge from aggressive tank mates or feeling unwell. It's important to check the water parameters, ensure a proper environment, and monitor for any signs of disease. If the behavior persists, consider consulting an aquatic veterinarian or an experienced aquarist.