Why Did My Fish Die? A Calm Checklist After a Sudden Loss

Finding a dead fish is a bad feeling, especially if it was fine yesterday. The urge is to grab a bottle of medicine and pour it in. Hold off. Most sudden deaths come down to water, and a few minutes of checking will usually tell you what happened and whether the others are at risk.

Step 1: Remove the fish

Take it out right away. A dead fish breaks down fast and spikes ammonia. If others might be sick, look it over first: white spots, fuzzy patches, red streaks, a swollen belly, torn fins.

Step 2: Test the water

This is the most important step. Test:

• Ammonia: should be 0.
• Nitrite: should be 0.
• Nitrate: ideally under 40 ppm.
• pH: compared with what it usually is.
• Temperature.

If ammonia or nitrite is above zero, that's very likely your answer. Do a 25 to 50% water change with dechlorinated water at the same temperature. See reading your test kit.

Step 3: Ask what changed

Most deaths follow a change. In the last few days, did you:

• Add a new fish? It may have brought in a disease. See how to quarantine new fish.
• Do a big water change without dechlorinator, or with water much warmer or colder than the tank?
• Clean the filter in tap water? That can kill the good bacteria and crash the cycle.
• Start a new tank in the last month or two? It may not be fully cycled.
• Overfeed, or go away and have someone else feed?
• Use sprays near the tank: air freshener, bug spray, cleaners, paint fumes?
• Put your hands in with lotion or soap on them?
• Lose power? Cold water and no filter flow can kill. See keeping a tank warm in a power outage.

Step 4: Check the equipment

• Heater: stuck on (water too hot) or dead (too cold)? Check with a separate thermometer.
• Filter: running and moving water?
• Air pump or air stone working?
• Lid: did the fish jump? Check the floor behind the tank.

Step 5: Watch the others

Look closely at the rest of the fish for a day or two:

• Gasping at the surface: oxygen or ammonia.
• Clamped fins or hiding: stress or illness.
• Rubbing against rocks: parasites or irritated gills.
• White spots like salt grains: ich.

Treat only once you know what you're treating.

Sometimes it's just age

Fish don't live forever. Many small community fish live 2 to 5 years. If one old fish dies and the water tests fine and the others look healthy, it may simply have been its time.

What not to do

• Don't add medicine "just in case." Many medicines stress healthy fish and can harm shrimp and snails.
• Don't do several big changes at once. One good water change, then watch.
• Don't add new fish until you know what happened.

Sources (2)

Aquarium Science: Fast Fish Deaths · Practical Fishkeeping: New Tank Syndrome

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