Resting or struggling?
Brief rest with normal posture and a normal response to activity differs from prolonged bottom-sitting. A fish lying on its side, breathing rapidly or unable to rise needs prompt attention.
Possible causes
Poor water, unsuitable temperature, injury and systemic illness can all reduce activity. A fish that tries to rise but sinks again may have a buoyancy disorder. Position alone cannot identify a pathogen.
Useful observations
Record when the behaviour began, recent water changes, new fish and filter maintenance. Check the fish from the side and above for swelling, fin damage or raised scales. Share a short swimming video and water readings with an aquatic veterinarian.
When to get professional help
Seek urgent help for severe breathing difficulty, bleeding, raised scales or inability to stay upright. Record exact water results and recent changes. A separate hospital aquarium must also be adequately filtered and aerated; an uncycled bowl can make the problem worse.
Frequently asked question
Should I medicate before testing the water?
Check water and oxygen promptly. Treatment should follow the likely cause; avoid mixing remedies without qualified guidance.
Sources and further reading
General husbandry and health references. Individual fish and local conditions vary.