Goldfish Connection indicates that goldfish sit at the bottom of an aquarium because of high or low levels of certain chemicals, such as ammonia or nitrates, in the water. A low pH level can also cause goldfish to settle on the bottom of the tank.
When a pet owner realizes that all the fish are swimming at the bottom of the tank, it is important to test and adjust water conditions as needed. Undesirable levels of chemicals can lead to serious illness and fatality in goldfish. Reducing feedings and adding salt help reduce ammonia and nitrate levels, according to Goldfish Connection.