Guide
Suddenly losing fish - what to do
Step-by-step response when fish die unexpectedly, including when your base readings look fine.
- Remove the dead fish immediately: a decomposing body can spike ammonia within hours and spread disease to surviving tank mates.
- Test ammonia, nitrite, nitrate, pH, and temperature right now - write the numbers down so you can track changes over time.
- Zero ammonia and nitrite does not mean the water is fine: nitrate above 40 ppm, low oxygen, wrong temperature, or untreated tap water can all kill fish while your cycle looks healthy.
- Look at the survivors closely: clamped fins, rapid gill movement, white spots, bloating, or unusual swimming posture all point toward specific causes.
- Do a 20-30% water change with dechlorinated water before doing anything else - it buys time regardless of the cause.
- Do not add medication blindly: treating for the wrong thing stresses surviving fish and can crash your cycle.
Start here: the first 30 minutes
Finding a dead fish is stressful, and the instinct is usually to do something dramatic - dose medication, do a huge water change, add new fish to replace the lost ones. Resist all of that. The first 30 minutes are about gathering information, not reacting.
Fish net out the body straight away. Then run a full parameter test: ammonia, nitrite, nitrate, pH, and temperature. If you have a GH/KH test kit, use it too. Write every number down with the date and time. Patterns you can track over days will tell you more than a single reading ever can - this is exactly what the parameter logging in App-aquatic is built for.
While the tests are running, watch the tank. Note which fish are hiding, which are at the surface, whether anyone is being chased. You are looking for a pattern, not just a body count.
Then do a 20-30% water change with temperature-matched, dechlorinated water. Even if your numbers look fine, a partial water change dilutes anything you might have missed and gives the remaining fish a better environment while you work out what happened.
Do not do a large emergency water change of 50% or more unless ammonia or nitrite is dangerously high (above 1-2 ppm). A sudden large change can shock fish that have already acclimatised to the current conditions, and it can temporarily destabilise your biological filter.
What your test results are actually telling you
Ammonia or nitrite above zero
This is a cycle problem. Your biological filter is not processing waste fast enough, and the fish are being poisoned. Causes include: a new tank that was never properly cycled, overstocking, overfeeding, a filter that was cleaned too aggressively with tap water (which kills the beneficial bacteria), or medication that wiped out the bacterial colony.
Even 0.25 ppm of ammonia is harmful over time. At 1 ppm and above, fish will die - often suddenly, because stress weakens their immune system and makes them susceptible to opportunistic infections. See our guide on the nitrogen cycle explained if you are not confident your tank is properly established. If you have never cycled before a fish was added, read how to cycle a fish tank.
Fix: daily 25-30% water changes until ammonia and nitrite are consistently zero. Reduce feeding. Do not clean the filter. Do not add more fish.
Ammonia and nitrite are zero - still losing fish
This is where most guides stop being useful. "Your water looks fine" is not an answer when fish are dying. Zero ammonia and nitrite means your cycle is working, but there are several other things that kill fish in cycled tanks.
The hobby often treats zero ammonia and nitrite as proof that water quality is not the issue. It is not. Nitrate, dissolved oxygen, temperature stability, and water source problems all kill fish in tanks that test "clean." Do not stop investigating once you see those two zeros.
Hidden water quality problems
Nitrate
Nitrate builds up silently in established tanks. Most freshwater fish tolerate up to 40 ppm reasonably well, but sensitive species - discus, altum angelfish, many wild-caught fish - start showing stress above 20 ppm. At 80 ppm and above, even hardy fish like platies and danios begin to suffer. Long-term high nitrate suppresses immune function, so fish do not die immediately but become prone to infections and stop healing from injuries.
Check your tap water nitrate level too. Some municipal supplies run at 20-30 ppm straight from the tap, which means water changes are doing less than you think. Read aquarium water parameters for species-specific safe ranges.
Dissolved oxygen
A tank can look crystal clear and test perfectly for ammonia, nitrite, and nitrate while being dangerously low on dissolved oxygen. Signs: fish gasping at the surface, hanging near filter outlets, or hovering at the top at night. Causes include high temperature (warm water holds less oxygen), overstocking, excessive plant die-off at night, or a blocked/slowed filter reducing surface agitation.
Fix: increase surface movement immediately. Point a filter outlet toward the surface, add an air stone, or drop the water level slightly so the filter return breaks the surface. In warm weather this can be urgent - at 30°C (86°F), oxygen saturation drops significantly compared to 24°C (75°F).
Temperature
Most tropical freshwater fish need 24-27°C (75-81°F). A heater stuck on full, a heat wave pushing room temperature up, or a heater that has failed can all shift temperature outside safe range fast. Fish can tolerate gradual change but die from rapid swings. Check your thermometer - heater thermostats fail more often than people expect, and a digital thermometer placed at the opposite end of the tank from the heater will catch problems a built-in display misses.
Chlorine and chloramine
If you recently did a water change with untreated tap water, chlorine or chloramine could have caused gill damage that killed fish within 12-24 hours. This is more common than people admit - a dechlorinator bottle runs out, someone else in the household does a top-up without knowing the protocol, or a new municipal treatment changes the supply. Always keep a spare bottle of dechlorinator. If chloramine is used in your area (check with your water supplier), make sure your dechlorinator is rated for chloramine - standard sodium thiosulfate does not neutralise it.
Adding tap water directly during an emergency "quick top-up" is one of the most common causes of unexplained fish deaths in otherwise well-maintained tanks. The damage to gill tissue from even brief chlorine exposure is not always immediately visible - fish may seem fine for several hours then die overnight.
Disease
If water parameters check out, disease is the next suspect. The difficulty is that many diseases kill before obvious symptoms appear, especially in fish that were already stressed.
What to look for on surviving fish
- White spots (1 mm, salt-grain size): almost certainly Ichthyophthirius multifiliis (ich). Highly contagious. Treat the whole tank.
- Clamped fins, pale colour, lethargy: bacterial infection, poor water quality stress response, or early-stage parasites.
- Bloating, pinecone-scale appearance: dropsy - usually a sign of internal organ failure. Difficult to treat at this stage.
- Rapid gill movement, scratching on surfaces: gill flukes or other external parasites, or dissolved oxygen problems.
- Wasting away while still eating: internal parasites, most commonly in wild-caught or newly imported fish.
- Sudden death with no visible symptoms: viral infections, internal bacterial septicaemia, or water quality causes that have already been corrected.
If you suspect disease, move affected fish to a hospital tank before treating. Medicating the main tank can wipe out your beneficial bacteria, remove carbon from your filter before dosing, and avoid combination treatments unless you have diagnosed multiple problems - stacking medications adds unnecessary chemical stress.
A bare-bottom 20-40 litre hospital tank with a sponge filter seeded from your main tank is one of the most useful pieces of equipment a fishkeeper can own. It costs almost nothing to set up and run between uses, and it lets you treat sick fish without destroying your main tank's biology.
Aggression and stress
Chronic bullying kills fish - not quickly from a single wound, but through sustained stress that suppresses the immune system and prevents access to food. A fish that is being harassed will stop eating, hide constantly, and eventually develop infections from stress-related immune failure or direct physical injury.
The fish doing the chasing is not always obvious. Some species are territorial only during breeding, or only at feeding time. Others are peaceful in larger groups but turn on individuals when kept in the wrong numbers - many schooling fish become nippy and aggressive when kept in groups of fewer than six.
Watch your tank during feeding specifically. If one fish is monopolising food and others are avoiding the feeding area, that is a problem. Add a second feeding spot at the other end of the tank, increase hiding places, or rehome the aggressor. Our free stocking calculator can help you check whether your current mix is compatible before adding more fish.
Most stress-related deaths occur within 72 hours of a trigger event - new fish added, tank moved, large water change, or a temperature swing. If you can identify something that changed in the three days before losses started, you have likely found your cause.
New fish introductions
Adding new fish without quarantine is the single fastest way to introduce disease into a healthy tank. Most pathogens have incubation periods of 1-2 weeks, meaning a fish can look perfectly healthy in the shop and develop ich, velvet, or bacterial infection after you have introduced it to your established fish.
A 4-week quarantine in a separate tank is the standard recommendation. Even 2 weeks catches most fast-developing infections. If you cannot quarantine, at minimum float the bag for temperature acclimatisation, then slowly mix tank water into the bag over 30-45 minutes before releasing - this reduces osmotic shock, though it does nothing to prevent disease transmission.
What to do if you keep losing fish despite everything
If fish keep dying and you cannot find a cause, consider getting your tap water tested by a lab rather than a test kit - kits degrade, give false readings with certain water chemistry, and cannot detect heavy metals. Copper from old plumbing has killed entire tanks. Some tap water supplies have periodic treatment events that introduce temporary spikes. An independent test costs around £10-20 and rules out the water source entirely.
Also review your stocking honestly. Overstocked tanks run at the edge of their biological capacity - any additional stress tips them over. Use the guide on water change frequency to check whether your current maintenance schedule matches your stocking load.
Sometimes fish die and you never find out why. A fish that arrived with internal damage from poor transport, a genetic condition, or a viral infection that leaves no external signs will die regardless of how well the tank is maintained. If your parameters are solid, your other fish are healthy, and you have ruled out the obvious causes, accept that not every loss has a diagnosable answer - and focus on making conditions as good as possible for the survivors.
Track your parameters and get care reminders with App-aquatic.
Get the free appMy ammonia and nitrite are both zero but fish keep dying - what is going on?
Zero ammonia and nitrite rules out a cycle crash but does not rule out water quality problems. Test nitrate (aim under 40 ppm), check temperature stability, and confirm you are using dechlorinator for every water change. Also look closely at surviving fish for disease symptoms and watch feeding time for signs of aggression.
How long after a water change can fish die from chlorine exposure?
Gill damage from chlorine or chloramine can take 12-24 hours to become fatal. Fish may appear normal for several hours before showing respiratory distress. If you did a water change in the 24 hours before fish started dying and are not certain dechlorinator was used, treat this as the likely cause and do a 30% water change with properly dechlorinated water immediately.
Should I do a large water change when fish are dying?
A 20-30% change is almost always the right first response. A 50%+ emergency change is only appropriate if ammonia or nitrite is dangerously elevated (above 1-2 ppm). Large sudden changes can shock already-stressed fish and destabilise your filter bacteria, making things worse.
One fish keeps dying but the others look fine - what does that mean?
If only one species or individual is dying while others are unaffected, the cause is more likely to be species-specific water parameters (wrong pH or hardness), aggression targeted at that fish, or a disease that species is more susceptible to. It can also mean that individual was already compromised before you bought it. Check whether the dying fish is being bullied, and verify its specific water parameter requirements against your tank readings.
Is it normal to lose fish when a tank is new?
Losses in a new tank are almost always caused by an incomplete nitrogen cycle. Ammonia and nitrite build up before enough beneficial bacteria have colonised the filter to process them. This is called "new tank syndrome." You can avoid it by cycling the tank without fish first - read our guide on how to cycle a fish tank before adding any livestock.
More guides · Why did my fish die suddenly? · Quarantine new fish · Water parameters