Goldfish sit at the bottom of a tank when they are not well, often because toxins have built up in the water. Goldfish are usually active, and lethargy is a sign of illness.
Maintaining good water quality is essential for proper aquarium care. In the enclosed, finite space of a fish tank, toxic elements, such as ammonia and nitrates, can build up in the water causing disease and killing fish. The only way to detect and control toxins is to perform regular chemical tests on the water and to perform gradual water changes to remove impurities and maintain the proper water parameters.