Why are my fish dying? A test-first checklist to find the cause

The most likely cause is a water problem you can't see: ammonia or nitrite from an uncycled, overstocked or overfed tank, chlorine or chloramine from untreated tap water, or a sudden change in temperature or pH. Test ammonia, nitrite, nitrate, pH and temperature before you treat anything, and if ammonia or nitrite is above 0 ppm, change 50% of the water with dechlorinated, temperature-matched water and stop feeding for a day or two (if the tank has gone months without a water change, make small daily changes instead). If the water tests clean, check tank mates, disease signs and possible poisons, and see an aquatic vet if fish keep dying with no clear cause.

When fish start dying, test the water before you buy any medicine. The usual killers, such as ammonia, nitrite, chlorine and temperature swings, are invisible in clear water, and medicine dosed into toxic water only adds stress. Work through the steps below in order.

What to do right now

Remove dead fish. Check behind decor, inside caves, at the filter intake and on the floor around the tank. Fish decompose fast in warm water and add ammonia. If a vet might examine the body, bag it and refrigerate it, but don’t freeze it.
Test the water. Test ammonia, nitrite, nitrate, pH and temperature, plus KH if you can. Liquid kits give clearer readings at low ammonia levels than strips do. If a fish store tests it for you, ask for the numbers, not just “it’s fine”.
Change 50% of the water if ammonia or nitrite reads above 0 ppm. If only nitrate is above 40 ppm, change 25–30% every 2–3 days until it drops below 40. Use a dechlorinator that handles chloramine, dosed as the label directs, and match the new water to within 2 °F (1 °C) of the tank. If the tank hasn’t had a water change in months, read the old tank syndrome section first.
Stop feeding for a day or two. Less food means less ammonia, and healthy adult fish can safely go a few days without food.
Add surface movement. Aim the filter outflow at the surface or add an air stone. UF/IFAS recommends keeping oxygen at 5 mg/L or more, and warm water holds less. Gasping at the surface is the classic sign of too little, often worst before the lights come on, because plants use oxygen overnight.
Hold off on everything else. No medicine, no new fish and no filter cleaning until you know the cause.
Write down what changed in the past week: new fish, a water change, decor, a medicine, a power cut or a spray used nearby. The cause is often on that list.

How to read your test results

Test	Healthy	Problem	Usual causes
Ammonia	0 ppm	Any reading above 0	Uncycled tank, overstocking, overfeeding, a dead fish, a filter that was cleaned or stopped
Nitrite	0 ppm	Any reading above 0	Tank still cycling, or the filter bacteria were set back
Nitrate	Under 40 ppm	Above 40 ppm; above 80 ppm is serious	Too few water changes for the stock and feeding
pH	Steady, inside every species’ range	Outside the range, lower at each test, or an unplanned drop below 6.0	Buffer used up (old tank syndrome), new rock or wood, CO2 overdose in planted tanks
KH	4–8 dKH (70–140 ppm) in most community tanks	About 3 dKH (50 ppm) or lower, or lower at each test	Buffer used up: filter bacteria slow down and pH can crash
Temperature	Inside every species’ range	Outside it, or a change of more than 2 °F (1 °C) in a day	Failed or stuck heater, hot room, cold water change

Ammonia is worse in alkaline water. Kits measure total ammonia, but only part of it is in the toxic free form, and that share rises with pH and temperature. By the standard formula behind the University of Florida (UF/IFAS) ammonia table, at 77 °F (25 °C) it is about 0.6% at pH 7.0 but about 5.4% at pH 8.0. UF/IFAS says free ammonia above 0.05 ppm can damage fish tissue, so in pH 8 water a 1 ppm reading is already at that line.

Nitrite stops the blood carrying oxygen. Fish gasp even in well-aerated water, and their gills can turn brown. UF/IFAS says it can be toxic at levels as low as 0.1 ppm, below the first color step on many hobby kits, so treat any visible reading as a problem. The Merck Veterinary Manual lists adding chloride, which makes nitrite less toxic, as part of the treatment. Aquarium salt (sodium chloride) supplies it but doesn’t remove nitrite, so keep changing water. If you use salt, follow the label and check that your fish, plants and snails tolerate it. The water parameters guide covers every reading in more detail.

Match the pattern to the cause

How and when fish die narrows the list quickly.

What you see	Most likely causes	Check first
Several deaths in a tank set up less than 8 weeks ago	New tank syndrome	Ammonia and nitrite
Deaths within 24 hours of a water change	Chlorine or chloramine, temperature mismatch, pH or hardness jump, a big change in a long-neglected tank	Dechlorinator, new water’s temperature, when the last change was
Only new fish die, within days	Acclimation shock, shipping stress, disease from the store, water outside their range	Their species range, your acclimation method
One species keeps dying	Water outside its range, bullying, or a disease that targets it	Species range, tank mates
Fish gasping at the surface	Low oxygen (bigger fish usually struggle first), ammonia or nitrite, overheating, gill parasites	Temperature, surface movement, ammonia and nitrite
Fish, snails and shrimp die suddenly together	Poisoning	Sprays, soap, metals or medicines
Torn fins, hiding, fish missing	Fin nipping, bullying, predation	Watch at feeding time and lights-out
Losses one by one over weeks, fish thin	Chronic disease, high nitrate, slow bullying	Nitrate, body condition, then a vet
Long-established tank, pH lower at every test	Old tank syndrome	pH, KH, ammonia

New tank syndrome

Merck says new tank syndrome usually shows up within the first 6 weeks: fish suddenly turn sluggish, stop eating and often die. The filter bacteria, which can take up to 8 weeks to establish in a tropical tank, can’t keep up with ammonia and nitrite yet.

Test ammonia and nitrite every day, at the same time, before feeding.
After the first emergency change, change 25–50% of the water the same day ammonia or nitrite reads 0.25 ppm or more, and lean toward 50% if your pH is 8 or higher. If your tap water reads 0, a 50% change halves the reading.
Feed once a day, only what is eaten in about a minute.
Add no more fish until ammonia and nitrite stay at 0 for a week.

Established tanks relapse too. Rinsing filter media in tap water, replacing all of it at once, a filter switched off for hours or some medicines (Merck names copper and antibiotics) can kill enough bacteria to bring ammonia back. The fish tank cycling guide explains how to finish the cycle, and the ammonia guide covers emergencies.

Chlorine and chloramine from tap water

US water suppliers usually disinfect tap water with chlorine or chloramine to make it safe to drink, and both harm fish. The CDC warns that even low levels can make fish sick. Chlorine leaves water left standing for a few days, but chloramine does not, so treat every drop of new water with a dechlorinator labeled for chloramine, including top-offs. Dose it as the label directs.

A high dose can kill fish within hours of a water change, often with no warning signs. Merck lists lower, longer-term chlorine exposure as causing lethargy, extra slime, cloudy eyes and gill damage. The CDC notes that utilities may switch from chloramine to chlorine for a while to clear slime from pipes. Some suppliers announce these switches, so check your supplier’s notices if deaths follow a routine change.

Sudden changes and old tank syndrome

Fish handle a steady, slightly imperfect reading far better than a fast swing.

Temperature: Keep daily changes under 2 °F (1 °C). Read a thermometer every day, because a heater can fail or stick on.
pH and hardness: Match new water to the tank. Change tank chemistry over days, not hours.
New arrivals: Aquarium Co-Op notes that ammonia and CO2 build up in shipping bags, and once the bag is opened, pH rises and that ammonia turns toxic. Match the temperature, net the fish into the tank and discard the bag water. The acclimation guide has the full method.

Old tank syndrome catches people who top up evaporation for months instead of changing water. The buffer (KH) runs out and pH falls, often below 6.0. Ammonia is often above 10 ppm, Merck reports, but at that low pH almost all of it is in the far less toxic ammonium form. A big water change raises the pH and turns that ammonia toxic. Merck’s veterinary guidance is to bring the water back with small daily changes instead. Start at about 10% a day and work up toward 25% as the pH rises and ammonia falls, testing pH and ammonia before each change. Merck’s pet-owner page gives a different route: move the fish to a holding tank, then strip, clean and restart the tank as if it were new. An aquatic vet can help you choose.

Overstocking and overfeeding

Too many fish make more waste than the filter and water changes can remove. Warning signs are ammonia after feeding, nitrate passing 40 ppm within a week of a water change, cloudy water and morning gasping. Enter your tank and fish in the stocking calculator, which works from adult sizes, to see whether the tank is over capacity.

Overfeeding does the same, because uneaten food rots into ammonia. Feed what disappears in about a minute, once or twice a day, and remove leftovers after 5 minutes.

Incompatible tank mates

Bullying kills slowly. Stressed fish hide, stop eating and catch diseases they would otherwise fight off, and UF/IFAS lists aggression from other fish and crowding as stressors alongside poor water. Watch the tank for 10 minutes at feeding time and just after lights-out, when chasing and predation happen.

Fin nippers with long-finned fish. Tiger barbs are notorious nippers and will shred a betta’s fins.
Schools that are too small. Tiger barbs need a group of at least 8, and smaller groups nip far more.
Big mouths. A fish that can swallow a tank mate usually will in the end. Adult angelfish, for example, eat neon tetras.
Territorial fish. Male bettas fight each other in any size of tank.

Check any combination in the compatibility checker before you buy. To fix a mismatch, rehome or separate the aggressor, add plants and caves to break sight lines, or bring schools up to full size if the tank has room.

Disease signs to look for

UF/IFAS notes that poor water is a common stressor that comes before many disease outbreaks, so fix the water first. Then match what you see:

Sign	Likely problem	What to do
White grains like salt on body and fins	Ich (white spot)	Treat the whole tank; see the ich guide
Fraying, shrinking or white-edged fins	Fin rot, usually after poor water	Fix the water first; see the fin rot guide
Fine gold or rust-colored dust	Velvet, a serious parasite	Treat the whole tank promptly as the label directs, or see a vet
Fluffy white tufts, pale patches, a white band around the body or a growth on the mouth	Fungus on an injury, or columnaris, a bacterial infection that is easily mistaken for fungus	Isolate and fix the water; ask a vet to confirm which it is, because antifungals don’t treat columnaris
Rubbing on objects (flashing), clamped fins	Parasites or irritated gills	Test water, then look for spots; gill and skin flukes are too small to see, so a vet may need to check a sample
Red streaks, sores, ulcers	Bacterial infection or ammonia burns	Test ammonia, isolate the fish, see a vet
Swollen body with scales sticking out	Dropsy, a sign of serious internal disease	Isolate and see a vet; often fatal
Thin despite eating, bent spine	Chronic infection such as mycobacteriosis, or internal parasites	See a vet for a diagnosis

Use a separate hospital tank where you can, and one medicine at a time. Take activated carbon out of the filter while you treat, because it absorbs many medicines, and follow the product label exactly. Remove shrimp, snails and live plants before using copper, which Merck calls extremely toxic to invertebrates and plants. Copper and some other medicines also harm filter bacteria, so test ammonia during treatment.

Poisoning from sprays and household chemicals

Suspect poisoning when many fish die at once, often with snails and shrimp, while ammonia and nitrite test normal. Hobby kits can’t detect most toxins. Usual sources:

Bug sprays, air fresheners and flea treatments. NPIC says permethrin, an insecticide in many sprays and dog flea products, is highly toxic to fish.
Cleaning sprays, paint and varnish fumes. Air pumps draw room air straight into the water.
Soap, lotion or hand cream on your hands, or a bucket that was used for cleaning.
Rocks, ornaments, coins and other metal not made for aquariums.

Change 50% of the water now and again the next day, put fresh activated carbon in the filter, add aeration and remove the source. In future, spray in another room, or switch off the air pump and cover the tank until the room has aired. Rinse your hands well before they go in the tank.

When to see an aquatic vet

See a vet who treats fish when:

deaths continue after the water has tested clean for a week,
several fish die with the same unexplained signs,
a treatment used exactly as the label directs hasn’t worked,
fish have ulcers, swelling, raised scales, bulging eyes or are wasting away, or
you suspect a bacterial infection. Merck recommends sensitivity testing before any antibiotic is used, rather than guessing.

Ask your usual vet for a referral, or use the “Find a Fish Vet” map on the American Association of Fish Veterinarians website (fishvets.org), which covers North America. Call before you go, because each vet or lab has its own instructions. What to bring usually looks like this:

A live, sick fish if possible, in tank water in an insulated container with an air supply.
Or a recently dead fish, sealed in a bag and kept chilled in the fridge, not frozen. Fish break down quickly after death, and the University of Guelph’s fish lab accepts chilled bodies only within about 12–24 hours. Freezing limits what the vet can find, so only freeze it if they ask.
At least 1 quart (about 1 L) of tank water, the minimum Merck gives for a water test, in a separate clean bottle capped under water so no air bubbles are trapped, and kept cool.
Notes, photos or a short video: tank size and age, filter, water source, stock, recent additions, test readings, treatments used and when deaths started.

Some fish germs, including Mycobacterium, can infect people. The CDC advises wearing gloves for tank work, especially if you have cuts or scrapes on your hands, and washing your hands afterward. People with weakened immune systems should let someone else care for the tank where possible.

How to stop it happening again

Cycle the tank before adding fish, then add a few at a time.
Quarantine new fish for at least 30 days, which Merck gives as the minimum. A 10-gallon (38 L) tank with a sponge filter and heater is enough for most small fish.
Check stocking and compatibility before you buy.
Dechlorinate all new water and match its temperature.
Test nitrate weekly, keep it under 40 ppm, and count your fish daily.

Common mistakes

Treating before testing. Medicine won’t fix ammonia, and some medicines make it worse by harming filter bacteria.
Rinsing filter media under the tap. Chlorine or chloramine can kill much of the colony, and ammonia returns within days. Rinse media in old tank water.
Replacing dead fish straight away. The new fish walk into the same problem.
Mixing medicines or dosing “just in case”. It stacks stress on fish that are already weak, and Merck calls “shotgunning” fish with many different drugs a dangerous practice.
Using expired or damp test strips. A false “all clear” hides the real cause.

Gear mentioned in this guide

Liquid test kit (ammonia, nitrite, nitrate, pH) (opens Amazon in a new tab)Liquid kits are more accurate than strips. The API Freshwater Master Test Kit is the common choice. How to choose
Water conditioner (dechlorinator) (opens Amazon in a new tab)Removes chlorine and chloramine from tap water. Seachem Prime is a popular, concentrated choice. How to choose
Aquarium thermometer (opens Amazon in a new tab)Check it daily: heaters can fail stuck on or off. How to choose

Product links go to Amazon search results. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Disclosure.

Frequently asked questions

Why do my new fish keep dying?

New fish usually die from an uncycled tank, stress after shipping, a pH or temperature jump during acclimation, disease they brought from the store, or water outside their species' range. If it keeps happening, quarantine new arrivals for at least 30 days and check each species' needs before you buy.

Why are my fish dying when the water tests fine?

The usual ammonia, nitrite, nitrate and pH tests don't detect sprays, soap or metals, only some strips test for chlorine, and no test shows a problem that has already passed, such as an overnight heater failure. Once water is ruled out, look at bullying, disease signs and anything new near the tank, and ask a vet to examine a fish if deaths continue.

How quickly should I remove a dead fish?

As soon as you see it. Fish decompose quickly in warm water, adding ammonia, and other fish may eat the body and pick up whatever killed it. Count your fish daily so you notice a missing one hiding behind decor.

Should I change all the water if my fish are dying?

No. A 100% change can shock fish with a sudden temperature, pH or hardness swing. In a tank that gets regular water changes, a 50% change halves a dissolved toxin, and repeating it the next day brings it down further while keeping conditions close to what the fish are used to.

Can one sick fish kill the rest of the tank?

It can, if the cause is a contagious disease such as ich or a bacterial infection. Move a visibly sick fish to a separate hospital tank, but remember that the same water problem that made one fish ill is often stressing the others too.