swim bladder disease
When a fish is not moving, at the bottom of the tank, due to stressors or being scared. Stress can lead to swim bladder disease in some species, which can manifest resulting in lying at the bottom of the aquarium.
Chloho flushed her fish Rosey when she saw it still at the bottom of the tank, suffering from Swim Bladder Disease, a trauma induced by their new and beloved cat PollyAnne. "Not All Fish Float When They're Dead", said Chloho. "Yes they do", replied Rosey, last seen swimming magically in the ocean, "because all drains lead to the ocean', exclaimed Chloho's friends trying to make her feel better.