Why are my fish dying? A calm, step-by-step checklist to find the cause

When fish are dying, the cause is usually the water, not a disease, so the first thing to do is test ammonia, nitrite, pH and temperature before you buy any medicine. Take a breath: this page walks you through a short checklist, then a symptom table that points you to the right guide.

Most sudden losses in home aquariums come down to a handful of causes: ammonia or nitrite from an uncycled or overfed tank, low oxygen, a temperature swing, or something toxic getting in. Diseases are real too, but they're easier to sort out once you've ruled out the water.

How to identify it

Look closely at the sick fish, and at the healthy ones, then match what you see:

What you see	Likely cause	Guide
Gasping at the surface, fast breathing, many fish at once	Ammonia, nitrite, low oxygen, heat	Ammonia, too hot
Deaths in a tank set up in the last 1–8 weeks	New tank syndrome	Cycling
Fish darting or dying within hours of a water change	Chlorine, chloramine, temperature shock	Chlorine
Sudden distress after cleaning, spraying, or hands in the tank	Soap or chemical contamination	Soap and contaminants
White grains of salt on body and fins	Ich (white spot)	Ich
Fine gold or rusty dust, fast breathing	Velvet	Velvet
Ragged, shrinking fins with white or red edges	Fin rot	Fin rot
White fuzz on the mouth, pale saddle on the back, fast deaths	Columnaris	Columnaris
Pits or holes on the head; stringy white poop; weight loss	Hole-in-the-head	Hole in the head
Fast breathing and flashing, water tests fine, gill covers held shut	Gill flukes	Gill flukes
Red threads from the vent; thin belly	Camallanus worms	Camallanus
Swollen body, scales sticking out like a pinecone	Dropsy (organ failure)	Dropsy
Low pH, KH near zero, no water change in months	Old tank syndrome	pH crash
Thermometer low; sluggish fish on the bottom	Heater failure, cold night	Cold tank
Power was off for hours	No air, heat or filter	Power failure
Thermometer high; fish at the surface	Overheating, low oxygen	Fish tank too hot
Leftover food, cloudy smelly water	Overfeeding	Overfeeding
Milky, green or brown water	Bacterial bloom, algae, tannins	Cloudy water, algae

When it's a problem

All at once, or one by one?

Many fish dying within hours or a day: almost always something in the water: ammonia, chlorine, a toxin, low oxygen, or a temperature swing. UF/IFAS notes that in a low-oxygen kill, fish die at about the same time, often at night or before dawn, and larger fish are often affected first (UF/IFAS). Emergency: act now.
One fish every few days: more often a spreading disease, a slow water problem (nitrite, high nitrate, a pH drift), bullying, or a tank that's too small for its fish. Serious: work through the checklist today.
Only new fish dying: shipping stress, a disease they brought with them, or water too different from what they came from (for example, an old tank with a very low pH).
Only one species dying: that species' needs aren't being met (temperature, pH, diet, tankmates), or a disease that targets it.
Normal, don't worry: one old fish dying after years in a stable tank, or a single fish lost in the first days after purchase while the rest thrive. Still test the water, but don't panic.

Gasping at the surface

Fish gulping at the surface ("piping") are short of oxygen or have damaged gills. The Merck Veterinary Manual's table of environmental hazards links surface piping to low oxygen and nitrite poisoning, and lists ammonia, chlorine and gill parasites among other causes of breathing trouble. Always treat gasping as urgent: add an air stone or point the filter outlet at the surface first, then test. Note: bettas, gouramis and some catfish gulp air normally.

What to do right now

Add aeration. An air stone, or the filter outlet breaking the surface. It helps in almost every case and harms none.
Remove dead fish right away. Check caves, plants and behind the filter for any you can't see.
Test the water: ammonia, nitrite, nitrate, pH, and temperature. Write the numbers down with the time.
- Ammonia and nitrite should be zero in a cycled tank. Nitrite above about 0.1 mg/L is a concern (Merck).
- Nitrate is best kept low with regular water changes.
- pH should be steady and suit your fish; a big drop is a warning sign.
- Temperature should match your fishes' needs and yesterday's reading.
If ammonia or nitrite is above zero, do a 30–50% water change with dechlorinated, temperature-matched water, unless the pH is very low in a long-neglected tank (read the pH crash guide first).
Look at the fish with a flashlight: spots, dust, fuzz, torn fins, swelling, red streaks, the gills, the vent, the belly. Compare with the table above.
Think about the last two weeks. What changed?
- New fish, plants or snails added? (Disease.)
- New decor, rocks, gravel or wood? (Toxins, pH change.)
- A big clean, a filter clean or a new filter? (Lost bacteria, ammonia.)
- A water change: did you add conditioner? Was the temperature right? (Chlorine, shock.)
- A medicine or water treatment? (Filter damage, overdose.)
- A heat wave, cold snap, or power cut? (Temperature, oxygen.)
- Sprays, cleaning products, paint or air fresheners in the room? (Contamination.)
- More food than usual, a new food, or a holiday feeder? (Overfeeding.)
Stop feeding for a day while you work it out.
Don't add medicine yet unless you've identified a disease. Medicine in bad water often makes things worse.

What to do next

Follow the guide for the cause you found in the table.
Test daily for a week, and keep a short log of readings, water changes and losses. Patterns show up fast on paper.
If the water is fine and you can't see anything on the fish, internal problems (gill parasites, worms, internal bacteria) are more likely. This is when a vet or fish health lab really helps.
Don't stack medicines. Use one targeted treatment at a time, remove activated carbon during treatment, follow the label, and finish the course.
Take extra care with sensitive animals: shrimp, snails, plecos and other catfish, loaches and scaleless fish react badly to many medicines, salt and copper. Check every label.
Protect the healthy fish. If you have a spare tank, move clearly sick fish to it, and don't share nets between tanks.

Products that help

Liquid test kits for ammonia, nitrite, nitrate and pH: the first tool for any unexplained death.
KH test kit: shows whether your pH is about to crash.
Reliable thermometer, ideally digital with an alarm: catches heater failures and overheating.
Air pump and air stone: emergency oxygen; a battery-powered one also covers power cuts.
Water conditioner that removes chloramine and detoxifies ammonia: for every water change and emergencies.
Gravel vacuum and a fish-only bucket: clean water changes without soap residue.
Spare tank with heater and sponge filter: a hospital and quarantine tank.
Flashlight: for checking gills, vents, and velvet's gold dust.

Keep on hand before trouble starts: test kits, conditioner, an air pump, a spare heater, and a quarantine tank.

How to prevent it

Cycle new tanks before adding fish, and add fish slowly.
Quarantine every new fish for about 4 weeks.
Test the water weekly and change 20–30% regularly.
Feed lightly and remove leftovers.
Check the thermometer and count your fish daily.
Keep sprays and chemicals away from the tank.

Why it happened

In most cases, one recent change tipped the balance: new fish, a big clean, a missed conditioner dose, extra food, or a heat or power event. The checklist above is designed to find it.

When to get help

Get help if fish keep dying and you can't find a cause, if many fish die at once, or if you keep valuable or rare fish. A vet who sees fish can examine a live sick fish or a freshly dead one (chilled in a bag, not frozen) and look at gills and skin under a microscope. A good local fish store can test your water and your tap water. Bring your log of readings and changes. Honest limit: sometimes you won't find the cause, and that's not a failure. Clean water, quarantine and patience prevent most repeats.

In the game

This isn't a card in Multiple Tank Syndrome. Every problem card in the game is one of the causes in the table above, and the products in your hand are the game's version of the kit list.