A number of issues cause goldfish to engage in bottom sitting, including poor water conditions, ammonia, nitrates, nitrites, parasites and swim bladder disorder. Bottom sitting is when a goldfish sits at the bottom of an aquarium.
To prevent bottom sitting, do a partial water change on the aquarium at least once a week. Removing 15 percent of the water keeps ammonia, nitrate and nitrite levels at a healthy level for the goldfish. Check the goldfish often for any raised bumps that might be caused by parasites. Soak goldfish food in dechlorinated water before feeding so that the goldfish does not ingest oxygen, which can also cause bottom sitting.