Decide whether to act urgently.
Do not wait for another daily check if fish are struggling to breathe, losing balance or deteriorating quickly. Contact a veterinarian who treats fish and describe how many are affected. Normal surface visits differ between species, so compare the behaviour with the fish's usual pattern rather than diagnosing from one movement.
Use the professional-help page for North American and UK directory routes, or ask your regular vet for a fish-treating referral elsewhere.
Check life-support equipment immediately.
Confirm the filter and aeration are actually running and water is moving. Check the thermometer against the species' needs. Restore failed equipment safely. If oxygen shortage is suspected, add appropriate aeration or surface movement while seeking advice. A plug being switched on is not proof that its pump is working.
Measure and keep the context.
Test ammonia, nitrite, pH and temperature; include salinity for a marine aquarium and dissolved oxygen if available. Record exact results and units. Note recent fish additions, cleaning, power interruptions, feeding changes and products used. If you cannot test, ask a retailer for help while arranging clinical advice; do not postpone urgent help for a test kit.
Avoid a chain of guesses.
Do not combine medicines, add salt or raise temperature because a photograph resembles a disease page. Ask for an appropriate water-correction plan. In a long-neglected aquarium, a sudden large chemistry change can itself be dangerous. Move an unwell fish only into a suitable prepared system with professional guidance, not an unfiltered emergency bowl.
Sources: Merck Veterinary Manual ↗
Give the next person something useful.
Take a short video of breathing and swimming without chasing the fish. Write the species, aquarium age and size, affected numbers, onset and measured water results. Add every treatment already used. Ask the vet or diagnostic service how they want samples collected before taking or sending fish or water.
Sources: UF/IFAS ↗ Merck Veterinary Manual ↗
There is no diagnosis, medicine selection or dosing in this guide. Water problems and infection can coexist; a normal-looking reading does not rule out illness. Veterinary availability and sample procedures vary by location.
- RSPCA: Pet fish health ↗Changes in breathing, balance, appearance and activity warrant investigation; check water and contact a specialist fish vet. Surface breathing and unusual posture need species context.
- UF/IFAS: Introduction to fish health management ↗Daily observation, prevention, water-quality screening and case history for professional evaluation. Production/pond observations are adapted cautiously to home-aquarium checks.
- UF/IFAS: Dissolved oxygen for fish production ↗Gasping can accompany oxygen depletion; emergency aeration matters when oxygen is deficient. Pond-specific schedules and numerical thresholds are not transferred to every aquarium.
- OATA: Water quality in marine aquariums ↗Marine salinity, evaporation, freshwater top-up versus matched saltwater replacement, and additional reef chemistry. No universal coral targets or additive doses.
- Merck Veterinary Manual: Management of aquarium fish ↗Fishless cycling, quarantine with separate equipment, clinical history, and careful correction of long-neglected water. Clinical drug, salt and treatment protocols are outside this pack.
- OATA: Find a fish vet ↗UK-oriented route to fish veterinary help and the Fish Veterinary Society. Not an international emergency service or confirmation a practice accepts new cases.
- AAFV: Find a fish vet ↗Provider directory currently supports North America; availability must be confirmed with the practice.
- Fish Veterinary Society: Find a vet ↗UK list of vets willing to treat pet fish; not an emergency-dispatch or appointment service.
