Is My Fish Sick? What to Do in the Next 10 Minutes
By Yechiel Kuperman · 49 years with fish
Quick answer
If you think your fish is sick, tell a grown-up first, then watch the fish for one minute and check it from nose to tail: eyes, gills, body, fins and skin.
Write down what you see, count whether other fish look wrong too, and don't feed it or add any medicine yet.
A grown-up should test the water next, because most fish illness starts with the water, and the note you wrote helps a fish shop or vet work out the rest.
The numbers
| What to check | Metric | US | What it means |
|---|---|---|---|
| Watching time, every day | 1 minute | 1 minute | You can only spot a change once you know the fish's normal |
| Tropical tank temperature | 24–26 °C | 75–79 °F | Too cold slows a fish down. A sudden swing is worse |
| Ammonia and nitrite | 0 mg/L | 0 ppm | Anything above zero is the first thing to fix |
| White spot (ich) size | 0.5–1 mm | 0.02–0.04 in | Like grains of salt sprinkled on the body and fins |
| Ich life cycle in a warm tank | 3–6 days at 24–26 °C | 3–6 days at 75–79 °F | Spots vanish before the parasite is gone. Finish the treatment |
| Meals missed before it is a real signal | more than 1–2 days | more than 1–2 days | One skipped meal on its own is not an emergency |
| Quarantine for new fish | 30 days | 30 days | In a spare tank, so new fish can't bring illness to the others |
Do this — step by step
- Minute 1: tell a grown-up, and don't put anything in the tank.No food, no medicine, no salt yet
- Minute 2: watch the fish for one whole minute without tapping the glass.
- Minutes 3–4: check it nose to tail: eyes, gills, body, fins, skin.A torch helps you see the skin
- Minute 5: count the fish. Is it one fish, or several?Several at once points to the water
- Minute 6: read the thermometer and check the filter is running.
- Minute 7: write down what you saw, and the time.
- Minutes 8–10: help your grown-up test ammonia, nitrite and pH.You read the colours, they handle the kit
- Skip the next feed while you wait for answers.
- If the water is fine and the fish is still wrong, take your notes to a fish shop or fish vet.
What does each part of a sick fish tell you?
A fish can't say where it hurts, so you read its body instead. Go from the nose to the tail in the same order every time, and you won't miss anything.
Eyes first. Healthy eyes are clear and sit neatly in the head. Cloudy eyes can come from poor water or an injury. Eyes bulging outwards, called pop-eye, often go with an infection inside the body.
Then the gills, the slits behind the head. Watch them move. Fast, heavy breathing, or gill covers held wide open, means the fish is struggling to get oxygen. Water problems cause this, and so do tiny parasites on the gills.
Now the body. A fish that suddenly looks swollen, with its scales lifting away from the body like a pinecone, has dropsy. That is a sign of a serious problem inside, often the kidneys, and a grown-up should get expert help the same day.
Very thin fish, with a sunken belly, may not be getting enough food, or may have a problem inside that stops them using it. Compare the fish with its tankmates.
Then the fins. Healthy fins are held open. Fins clamped flat against the body mean the fish feels unwell. Fins with ragged, shrinking edges, sometimes white or red at the tips, usually mean fin rot, a bacterial infection that takes hold when the water is dirty.
Skin last, and a torch helps. White spots like grains of salt are ich, a parasite 0.5 to 1 mm across. A fine gold or rusty dust, easiest to see by torchlight, is velvet, another parasite. White fluffy tufts like cotton wool are usually a fungus growing on a scrape.
How it swims matters too. Rubbing against rocks means itchy skin or gills. Swimming sideways, upside down or in circles is urgent. Tell a grown-up straight away.
Some things look like illness and are not. Many fish rest near the bottom at night, and lots go pale while they rest, then colour up again when the light comes on. Look again in the morning before deciding.
Almost every sign on this list can start with bad water. That's why Yechiel's rule in the printed guide never changes. When a fish seems unwell, check the water first.
By the way, out in the ocean…
Octopus. One of the ocean's cleverest animals: eight arms, three hearts, and skin that changes colour to vanish in plain sight. An octopus changes colour on purpose. When your fish changes colour, it usually means something, so write it down.
Dolphin. Famous for intelligence and play — dolphins solve problems and call each other by name with signature whistles. Aquarium dolphins learn to show vets their flippers for check-ups. Your fish can't, so your eyes do the nose-to-tail check.
Madame Molly’s Logbook Card
Molly · the Peacekeeper, breathes first
Madame Molly has calmed a great many worried visitors to the glass, and she always begins the same way. Breathe first. A frightened keeper taps, nets, pours and doses, and the fish ends up with two problems instead of one.
So breathe, fetch your grown-up, and look slowly from nose to tail. Write down what you see. Madame Molly finds that a child with a pencil and a calm minute is worth more to a sick fish than anything in a medicine bottle.
What goes wrong when a kid thinks a fish is sick
Keeping it a secret
Some children worry they caused it, or that they will get into trouble, so they wait. Most fish illnesses are easier to help early, and harder every day they are left.
Telling a grown-up straight away is the most useful thing you can do.
Netting the fish out for a closer look
Chasing a fish with a net stresses it and can rub off the slimy coat that protects its skin. A sick fish has little energy to spare.
Look through the glass instead, with a torch if you need one.
Giving it extra food to feel better
It is a kind idea. A sick fish often won't eat, and the food sinks, rots and makes the water worse.
Skip a feed. Healthy fish manage a day without food easily.
Adding medicine or salt from the cupboard
Medicines are made for particular illnesses, and some harm the filter bacteria. If the real problem is the water, they make things worse.
Test the water first, then let a grown-up decide what to use.
Waiting for a second sign
Some signs can't wait: a fish swimming sideways, a swollen body with lifted scales, or several fish in trouble at once.
Any of those means a grown-up today, and expert help if the water is fine. Bring your notes and the test numbers, because a fish shop or vet can do far more with them than with a worried description.
Questions we get asked
How can I tell if my fish is sick or just sleeping?
A resting fish sits still near the bottom or among plants, often a little paler, and wakes up and colours up when the light comes on. A sick fish stays still in daylight, breathes fast, clamps its fins or won't eat. If in doubt, watch again in the morning.
Will white spot spread to my other fish?
Yes, it can. White spot, or ich, is a parasite that breeds in the tank and attacks other fish, so the whole tank is treated, not just one fish. Treatment has to last past the day the spots vanish, because the parasite is still breeding when you can't see it.
Why are my fish's fins ragged?
Ragged fins with white or red edges that get shorter are usually fin rot, a bacterial infection that takes hold in dirty water. Torn fins with clean edges may be nipping from another fish. Either way, test the water, and watch who swims near the injured fish.
Why does my fish look swollen with its scales sticking out?
That is called dropsy. It is a sign of a serious problem inside the fish, often the kidneys, and fluid building up under the skin lifts the scales. It is hard to treat once the scales stick out, so a grown-up should ask a fish vet or aquarium shop the same day.
Why is my fish lying at the bottom of the tank?
Some fish, like corydoras, live on the bottom, and many fish rest there at night. A fish that normally swims but lies on the bottom in daylight, breathing fast or with clamped fins, is unwell. Check the thermometer and ask a grown-up to test the water.
Can I touch my sick fish?
No. Handling hurts a fish's protective slime coat, and tank water can carry germs that make people ill. The CDC says to wash your hands after any tank job, and children under five shouldn't handle tank equipment. A grown-up with clean hands or gloves should do any moving.
What should I tell a grown-up about my sick fish?
Say which fish it is, what you saw, and when you first noticed. Mention anything that changed recently, like new fish, a water change or a hot day. Your written notes from the nose-to-tail check are exactly what a fish shop or vet will ask for.
Should I move my sick fish to another tank?
Only if a grown-up has a spare tank that is already set up, with a running filter, heater and conditioned water. A bowl of fresh tap water will make things worse. For most problems, fixing the water in the main tank is the first step.
Sources
- Aquarium Basics, Part 9Kuperman, Y. Aquarium Basics: Warning Signs and Golden Rules (Part 9 of 9). Yechiel's Fishery Farm care guide (PDF). View source
- UF/IFAS FA006Francis-Floyd, R., Yanong, R. P. E. & Pouder, D. B. Ichthyophthirius multifiliis (White Spot) Infections in Fish. University of Florida IFAS Extension, FA006. View source
- Fin rotTree of Life Exotics veterinary client resources. Fin Rot in Fish. View source
- DropsyTree of Life Exotics veterinary client resources. Dropsy. View sourceNote: Dropsy is a sign of disease inside the body, not a single disease, and the outlook once scales lift is often poor.
- VelvetTree of Life Exotics veterinary client resources. Velvet Disease (Oodinium spp.) in Aquarium Fish. View source
- cdc-fishCenters for Disease Control and Prevention. Fish: Healthy Pets, Healthy People. View source
- Merck Vet ManualFrancis-Floyd, R., Yanong, R. P. E. & Petty, B. D. Environmental Diseases of Aquatic Animals in Aquatic Systems. Merck Veterinary Manual. View source
About the author
Yechiel Kuperman
Marine Biologist & Aquaculture Mentor
Yechiel has kept fish since he was four — 49 years with tanks — and 30+ years running farms. He mentors fish farmers internationally.
He writes the Yechiel's Fishery Farm books, in which the biology is real and the fish have opinions.
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