When a fish lies at the bottom of the tank, it typically means the fish is in its last stages of life. The fish no longer has the strength to swim.
It is important for dying fish to be promptly removed and quarantined to protect any additional fish from disease. Draining the fish tank and refilling it with fresh water helps to remove additional germs and bacteria.
Another reason for fish to lie at the bottom of the tank is a strong water current. Strong water currents can cause exhaustion for the fish, due to the fact that the fish has to work extra hard to move around the tank.