Quick answer
Stop feeding, increase aeration, test water immediately, remove dead animals and review every recent change. Treat the cause only after separating water-quality, toxin and infectious possibilities.
What to check first
- Test ammonia, nitrite, pH and temperature
- Check heater and electrical equipment
- Review sprays, cleaners and water changes
- Look for similar signs among survivors
Write down the test results, recent maintenance, new livestock, feeding changes and equipment faults. A short timeline prevents repeated guesses and makes patterns easier to see.
A practical action plan
- 1. Perform a conditioned water change when contamination or waste is likelyRecord what changes and retest before making another major adjustment.
- 2. Use fresh activated carbon for suspected chemicals where suitableRecord what changes and retest before making another major adjustment.
- 3. Quarantine new or affected stockRecord what changes and retest before making another major adjustment.
- 4. Contact a veterinarian if losses continueRecord what changes and retest before making another major adjustment.
What to avoid
Do not dispose of evidence before documenting it or add several medications together.
When it is urgent
Continuing deaths, neurological signs or suspected toxins requires urgent professional help.
How to prevent it next time
Use stable routines: quarantine additions, test before and after major changes, maintain backup aeration, keep equipment accessible, and record enough detail to compare today with the aquarium’s normal baseline. Prevention is usually cheaper and safer than emergency correction.
Research and review
This is original AquaNexus writing prepared for practical aquarium use. The husbandry framework was checked against authoritative veterinary, safety, scientific or specialist references.
Test rather than guess and make changes at a pace the livestock can tolerate. This guide does not replace aquatic veterinary advice. Follow product labels and local electrical-safety requirements.