A tropical fish constantly resting on the gravel is a serious sign of distress. Possible causes include disease, parasites, swim bladder disorder, constipation, poisoning from poor water quality, incorrect temperature, stress, old age, or lack of oxygen.
Observe the fish for rapid breathing, clamped fins, or visible sores. Immediately test the water for ammonia, nitrite, nitrate, pH, and temperature. If possible, isolate the fish in a quarantine tank. Based on your findings, treat with medication or adjust the environment, and consult a fish veterinarian if you are unsure.
Acting quickly can significantly improve the fish's chances of recovery.