How to Tell If Your Fish Is Sick: 9 Warning Signs
By Yechiel Kuperman, marine biologist · 49 years keeping fish
Quick answer
A fish that is unwell usually stops eating, holds its fins clamped against its body, hides, darkens, rubs against things, or hangs at the surface gulping.
None of those signs tells you which illness it is, because all of them can also be caused by the water itself.
So test the water first — ammonia, nitrite, pH and temperature — before you add any medicine.
Medicine given to a fish in bad water usually makes things worse.
The numbers
| What to test | Healthy | On a US test kit | What happens past it |
|---|---|---|---|
| Temperature (tropical community tank) | 24–28 °C | 75–82 °F | A sudden swing is more dangerous than a steady wrong number |
| Dissolved oxygen | above 5 mg/L | above 5 ppm | Fish gulp at the surface below roughly 2 mg/L |
| Un-ionised ammonia (NH₃) | 0 mg/L | 0 ppm | Gill damage from 0.05 mg/L; about 2 mg/L kills sensitive fish |
| Nitrite (NO₂⁻) | 0 mg/L | 0 ppm | Turns blood brown — fish suffocate even in well-oxygenated water |
| pH | 6.5–7.5 for most community fish | same scale everywhere | Every 1 point up multiplies the toxic ammonia roughly tenfold |
| Nitrate (NO₃⁻) | under 40 mg/L | under 40 ppm | Slow, grinding stress rather than sudden harm |
Do this — step by step
- Stop. Do not put any medicine in the tank yet.Almost every rushed treatment starts here
- Go and get a grown-up. This is a two-person job.One person watches the fish, one handles the water
- Watch the fish for one whole minute and write down exactly what you see.Fins, breathing, colour, where it is sitting
- Test the water: ammonia, nitrite, pH and temperature.
- If ammonia or nitrite is above zero, change about a quarter of the water.Dechlorinated, and matched to the tank temperature
- Check the filter is actually running and the surface is moving.
- Stop feeding for twenty-four hours.Uneaten food is what made most of the ammonia
- Test again twelve hours later and write the new numbers beside the old ones.
- If the water is clean and the fish is still wrong, take your written numbers to an aquarium shop or a fish vet.The numbers are worth more than a description
Why does a sick fish look the same no matter what is wrong?
Because a fish has a very short list of ways to look unwell, and a very long list of things that can go wrong.
A fish breathes by pulling water across its gills. The gill is not tucked away inside the body like a lung — it is a thin, exposed, feathery surface with blood running just beneath it. That is what makes it so good at collecting oxygen. It is also what makes it the first thing to be damaged.
Ammonia damages it chemically. Nitrite poisons the blood behind it. Low oxygen starves it. Parasites physically graze on it. Four completely different problems, all landing on the same organ.
So the fish does the same four things every time: it breathes harder, it stops eating, it clamps its fins, and it goes somewhere quiet. That is the whole vocabulary.
Ammonia is worth understanding properly, because it is the one people get wrong. Ammonia in water exists in two forms, and only one of them is really dangerous — the un-ionised form, NH₃, which is roughly a hundred times more toxic than the other.
What decides the balance between them is pH. At pH 7 and 20 °C, only about 0.4% of the total ammonia is in the dangerous form. At pH 9, it is over 28%.
That is the same test result meaning two entirely different things. It is why any advice that says "keep ammonia under X" without mentioning pH is simply incomplete.
Nitrite works differently and is stranger. It converts haemoglobin — the molecule that carries oxygen — into methaemoglobin, which cannot carry oxygen at all. The blood turns brown.
A fish poisoned by nitrite suffocates in water that is full of oxygen. This is why gasping at the surface is not proof of an oxygen problem, and why the meter alone can mislead you.
Ich, the white-spot parasite, hides a similar trap. The visible white dot is a parasite burrowed under the fish's skin, where medicine cannot reach it. Only the free-swimming stage, after it drops off, can be treated.
So when the spots disappear, the infection is not over. The parasites have simply left to reproduce — one of them can become over a thousand.
That is the through-line of this whole page. The sign you can see is rarely the thing that is happening. The water is where the answer usually is.
By the way, out in the ocean…
Axolotl. The internet's favourite smiling amphibian can regrow a whole lost leg — yet in the wild it is critically endangered. The same early-warning habit you're learning here is how axolotl keepers catch trouble before it gets serious.
Sea turtle. Ancient mariners — their kind has crossed the oceans for over 100 million years, migrating thousands of kilometres. Rescue centres check turtles the same way you just learned: appetite, posture, and how they swim.
From 49 Years at the Farm
Yechiel, asked what he tests first · July 2026
People always want a rule for this. They want me to say: test ammonia first, every time, and then you will know.
After forty-nine years I am not going to pretend that is what I do.
It depends entirely on why the fish looks wrong. Is it hanging at the top? Is it sitting on the bottom? Is it gasping?
Those are three different questions. They send me to three different tests first.
New arrivals are the same. Sometimes not one of them is ill. Sometimes every single one is. There are no rules. There is only nature.
So what I teach is not a checklist. A checklist tells you what to do before you have looked at anything.
Listen to what the fish is telling you. Listen with your eyes.
Then test. The kit confirms what the fish has already shown you — which is why watching for a minute comes before step four, and not after it.
Captain Cory’s Logbook Card
Corydoras · cleanup sergeant, gravel patrol
Right. Down here on the gravel I see everything, and I mean everything. I see who ate and who only pretended to. I see who is sitting in the corner with their fins pinned flat like a folded umbrella.
Here is my standing order, and I do not repeat it twice. Look at your fish every single day when nothing is wrong. That is the whole trick. You cannot spot a change if you never learned the normal.
One minute a day. That is all I ask. Barbels down, eyes open, and no panicking.
What goes wrong when people try to help
Reaching for medicine before testing the water
This is the most common mistake and it is made out of kindness. The fish looks ill, so something must be done, so something goes in the tank.
But most medicines are hard on an already-damaged gill, and several will kill the filter bacteria outright. If the cause was bad water, you have just made the water worse.
Test first. Always. The test kit is the cheapest thing you will ever buy for that tank.
Changing all the water at once in a panic
A huge sudden change swings temperature, pH and hardness all at the same moment — which is a second shock on top of the first.
A quarter of the tank, matched for temperature and dechlorinated, does the job without the shock. Repeat it tomorrow if you need to.
Rinsing the filter sponge under the tap
Chlorine in tap water kills the bacteria living in that sponge almost instantly, and those bacteria are the thing removing ammonia.
Squeeze it out in a bucket of old tank water instead. Take the dirt, keep the biology.
Trusting clear water
Ammonia and nitrite are both completely invisible. Water can look like glass and still be dangerous.
Clear is not the same as clean. Only a test tells you which one you have.
Stopping treatment the moment the white spots vanish
The spots disappearing means the parasites have dropped off to breed, not that they have died.
How long the cycle takes depends on temperature — in a warm tank it can be as little as three to six days. Finish the course the packet tells you to finish.
Questions we get asked
How do I know if my fish is sick?
Watch for a change from that fish's own normal: not eating, fins held tight to the body, hiding much more than usual, darkening, rubbing against objects, or hanging at the surface gulping.
Any one of them means test the water. None of them, on its own, tells you which illness it is.
Why is my fish gasping at the surface?
Usually low oxygen — but not always, and this is the one most people get wrong.
Nitrite poisoning stops the blood carrying oxygen, so a fish suffocates even in water that is full of it. Damaged gills do the same. Test nitrite as well as checking the filter.
How to tell if a fish is dying?
A fish close to death usually stops swimming upright, lies on the bottom or rolls onto its side, stops responding to movement outside the glass, and breathes either very fast or very slowly.
Get an adult now. Test the water, and be prepared that it may not be fixable.
Why is my fish not eating?
One skipped meal is not an emergency — most healthy fish can miss a day without harm.
Two or three days of refusing food is a real signal. It appears with ammonia exposure, with parasites, and with water that is too cold. Test the water and check the temperature.
Why is my fish rubbing against things?
Rubbing or flashing against gravel and rocks means the skin or gills are irritated.
In the fish-health literature this is most characteristically associated with external parasites, but harsh water chemistry irritates skin too. Test the water first, then look closely for spots or a dusty film.
My fish has white spots — what is it?
Most likely ich, a parasite that burrows under the skin, which is why each spot looks like a grain of salt sitting in the fish rather than on it.
It arrives on new fish, plants or equipment. It does not appear from nowhere in a closed tank.
Do peas cure swim bladder problems?
There is no published evidence for it. We looked, and found no study, no veterinary reference and no university extension source supporting the peas remedy.
It is popular advice, but it is folklore. A fish struggling to stay upright needs its water tested and an adult's help.
Can a sick fish get better on its own?
Often, yes — if the cause was the water and you fix the water. Fish recover from mild ammonia or temperature stress surprisingly well once the conditions are right.
An established infection is different and usually needs treatment. That is why testing first matters so much.
Is it my fault my fish died?
Usually not. Some fish arrive already ill from the shop, some are old, and some problems are invisible until they are advanced.
Noticing early is the kindest thing anyone can do for a fish, and you were the one looking. That counts for a great deal.
Sources
- UF/IFAS FA16Francis-Floyd, R., Watson, C., Petty, D. & Pouder, D. B. Ammonia in Aquatic Systems. University of Florida IFAS Extension, FA031. 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
- Lewis & Morris 1986Lewis, W. M. Jr. & Morris, D. P. (1986). Toxicity of Nitrite to Fish: A Review. Transactions of the American Fisheries Society 115(2), 183–195. View source
- UF/IFAS CIR920Francis-Floyd, R., Yanong, R. P. E. & Pouder, D. B. Ichthyophthirius multifiliis (White Spot) Infections in Fish. University of Florida IFAS Extension, FA006. View source
- Kramer & McClure 1982Kramer, D. L. & McClure, M. (1982). Aquatic surface respiration, a widespread adaptation to hypoxia in tropical freshwater fishes. Environmental Biology of Fishes 7(1), 47–55. View sourceNote: This study reports the gasping threshold in torr, not mg/L. The rough 2 mg/L figure used above comes from the veterinary and extension sources, not from this paper.
- Martins et al. 2012Martins, C. I. M. et al. (2012). Behavioural indicators of welfare in farmed fish. Fish Physiology and Biochemistry 38(1), 17–41. View sourceNote: Colour change is better evidenced as darkening than as fading. A fish also pales for entirely healthy reasons, such as matching a pale substrate, so paling is a poor diagnostic sign.
Want the original printed guide?
Download the PDFAbout the author
Yechiel Kuperman
Marine Biologist
Marine biologist. Forty-nine years in fishery and aquarium work.
Chief Scientist of the Israeli Ornamental Aquarium Society for fifteen years, and the developer of specialist fish strains supplied to European importers.
He writes the Yechiel's Fishery Farm books, in which the biology is real and the fish have opinions.
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