Why Do Fish Die in an Aquarium? Water Causes to Check

The short answer

The usual causes are ammonia and nitrite in a tank that has not finished cycling, tap water added without a conditioner, temperature swings, low oxygen, too many fish or fish added too fast, and sudden pH changes. API notes that “most water problems are invisible to the eye”, so test the water and read the thermometer first.

Practical Fishkeeping, a UK fishkeeping magazine, puts it plainly: “water quality, or failure of it, is the biggest cause of fish deaths”. The trouble is that you cannot see it. Aqueon, an aquarium equipment maker, warns that fish often adjust to slowly rising ammonia and nitrite and “may not show signs of distress until it’s too late”, and that “testing is the only way to know if your aquarium water is safe”.

Warning

If fish in your tank are sick or dying right now, test the water now (ammonia, nitrite, nitrate, pH and temperature) and contact an aquatic veterinarian. This guide covers water and setup causes only: fix any water problem the test shows, and leave identifying and treating illness to the vet. It covers freshwater aquariums: the water-quality pages it cites (Aqueon’s freshwater water-quality guide, API’s freshwater test kit) are written for them. For a saltwater or reef tank, follow your salt mix and test-kit makers’ instructions.

If fish are dying right now

Practical Fishkeeping’s advice is that “every time you find a dead fish you must test the water”, and to do it straight away. In order:

Remove the dead fish

The magazine explains that a body “will quickly rot in the warm, bacteria-laden water” and pollute it. Check behind decorations and near the filter intake for any fish you cannot account for.

Test ammonia, nitrite, nitrate and pH, away from the tank

Practical Fishkeeping suggests keeping a liquid test kit that covers all four; the API Freshwater Master Test Kit is one example, a liquid kit that API says checks “ammonia, pH, high range pH, nitrite and nitrate”. The magazine also warns to “never place the sample on top of the aquarium while you wait for the results. A spillage could prove deadly to your fish.” Use a kit that is in date, because Aqueon notes that test kits “can expire”. Write the numbers down.

Read the thermometer and check the equipment

Confirm the heater, filter and any air pump are running. In Practical Fishkeeping, writer Nathan Hill names “a crash of the filter” and “a stuck, overzealous heater” among the catastrophic causes of sudden losses. If the thermometer reads outside your fish’s range, bring it back slowly: Aqueon warns not to lower the temperature “more than 2 or 3 degrees F in a 4-hour period” and says to “always remember to make any fish temperature changes in your aquarium gradual”.

Change water if ammonia or nitrite reads above zero

Stop feeding first; Aqueon’s ammonia guide explains that fed fish “produce more waste”. The makers differ on size. Aqueon’s ammonia guide says to “change between 25 to 50% of the tank’s water” for any ammonia reading. Aqueon’s do’s and don’ts says to “avoid water changes of more than 25% as larger water changes can stress your fish and upset the balance in your aquarium”. API’s FAQ says “it is standard to perform a 25% water change” when cleaning, which it recommends every two weeks, and, for fish gasping at the surface, to check all water parameters and “if necessary, perform a 25% water change.” The sources differ, and none gives a single figure for every tank. Treat the new water with a conditioner before it goes in, match its temperature “as closely as possible” (Aqueon), and test again afterwards. Nitrite gets the same action: Aqueon’s cycling guide gives partial changes the job of making sure “ammonia and nitrites don’t reach toxic levels”. Our guide to why aquarium water changes matter covers routine changes.

Write it down and ask for help

Practical Fishkeeping suggests noting the results with your tank size, filter model, how long it has been set up and what you feed, then consulting an expert. For fish still alive but unwell, contact an aquatic veterinarian.

The usual causes, and how to check each

Cause	How to check	Equipment-side fix
Ammonia or nitrite (new or overloaded tank)	Ammonia and nitrite test; both should read zero	Feed less, partial water changes, add fish slowly
Chlorine or chloramine	Was every refill treated?	Water conditioner on all tap water
Temperature swings	Thermometer, checked regularly	Heater sized for the tank; match refill temperature
Low oxygen	Thermometer; is the aerator running?	An aerator (air pump and air stone)
Too many fish, or too fast	Ammonia and nitrite test before each addition	Fewer fish; a few at a time, a week apart
Sudden pH change	pH test of tank and new water	Slow acclimation; no quick pH products

Aqueon’s water-quality guide gives the line for the first row: “the only safe ammonia level is zero”, and the same for nitrite. It adds that ammonia “is more toxic at higher temperatures and pH levels above 7.0”. Our guide to why aquarium ammonia gets high covers the usual sources.

New tank syndrome: deaths in the first weeks

In a new tank, the bacteria that turn fish waste (ammonia) into nitrite and then into far less toxic nitrate are not established yet; Aqueon calls a new aquarium “a biological clean slate”. It names the result “New Tank Syndrome”: adding too many fish at once or overfeeding a new tank “will cause ammonia and nitrite to rise to unsafe levels”. Running the tank “for 24 to 48 hours after first setting it up is not cycling”, Aqueon adds.

How the first weeks usually go, by maker:

Aqueon: ammonia typically begins to rise “by the third day” after the first fish, nitrite rises by the second week, and “the whole process typically takes three to eight weeks”.
sera, a German maker of test kits and water care, describes the danger point as a nitrite peak, “usually between the 5th and 15th day”. On its scale any nitrite above zero means action is needed, and “at the latest at 1.0 mg/l, nitrite is harmful”. That is 1 ppm on US kits: API, a US test-kit maker, gives its readings in “parts per million (ppm) or mg/L”.
Interpet, a UK maker of test kits and treatments, says the filter bacteria “can take up to 2 months” to establish, and that new tank syndrome “can last anywhere between 2-12 weeks”.

Aqueon’s “safest way” to cycle a tank with fish:

Add just a few fish to start

Aqueon warns that stocking a tank full of fish right away “can spell disaster”.

Feed sparingly

Aqueon’s rule for a new tank is food once a day, all of it eaten “within two minutes”.

Test ammonia and nitrite every two to three days

Aqueon expects you may need to change “about 10 to 25% of the water every few days” while it cycles, with conditioned, temperature-matched water.

Add more fish only when both read zero

Then add a few at a time, as Interpet also advises.

The same spike can come back later. sera lists “a filter change or too thorough filter cleaning” as a trigger, because it removes many of the useful bacteria, and advises monitoring nitrite afterwards. Aqueon’s first-60-days guide handles filters by timing. In days 15 to 30, rinse solid waste off cartridges “as needed” and “do not disturb biological media at this time”. Change cartridges “after the first 30 days and then once a month thereafter”. Test nitrite after any filter change until it reads zero.

Untreated tap water: chlorine and chloramine

Whether your water utility uses chlorine or chloramine, the US Environmental Protection Agency (EPA), a federal agency, states that “chlorine and monochloramine must be neutralized or removed if used in aquariums”. Both can harm fish because they “directly enter their bloodstream through the gills”, and the EPA adds that they can also prevent the growth of a tank’s beneficial bacteria.

Leaving tap water out overnight does not fix chloramine: the EPA notes that “allowing water to sit at room temperature does not remove monochloramine”, and neither does boiling.

Treat every refill, top-offs included. Aqueon says to use a water conditioner “BEFORE you put new water in your aquarium”. Pick one whose label says it treats chloramine as well as chlorine; API’s FAQ, for example, says its Stress Coat conditioner “instantly removes chlorine and chloramines from tap water”. Our water conditioner roundup compares the options.
Keep a bucket for the tank only. Aqueon’s list of don’ts includes buckets and scrub pads “that have been used with soap or other chemicals”.
Watch what goes into the air and onto your hands. In Practical Fishkeeping, Nathan Hill lists flea spray, air fresheners and soap residue on hands among the household chemicals that can kill fish, and advises using hand soap and sanitizer after working in the tank, not before.

Temperature swings and low oxygen

Aqueon says tropical fish “do best between 75° and 80°F”, while goldfish and other cold-water species “prefer temperatures below 70°F”. It points out that temperature changes “happen much more rapidly in an aquarium” than in nature. Heat brings two more problems: water “holds less oxygen” as it warms, and above 90°F the tank’s beneficial bacteria start to die, which can set off an ammonia and nitrite spike.

Place the tank away from sunny windows, doors, vents and drafts

Aqueon names all of these.

Use a heater sized for the tank and an accurate thermometer

Aqueon advises checking an accurate thermometer regularly to make sure the heater is holding the right temperature. If the temperature drifts, see why an aquarium heater stops working.

Match the temperature of new water

Aqueon advises matching replacement water “as closely as possible” to the tank.

Cool slowly in a heat wave

Aqueon’s summer guide suggests floating a bag of ice or cold water and warns not to lower the temperature “more than 2 or 3 degrees F in a 4-hour period”. It also warns: “Avoid putting ice cubes directly into your aquarium unless you make them with reverse osmosis or dechlorinated water.” If your utility uses chloramine, make them from conditioned water: the EPA notes that reverse osmosis filters “do not remove monochloramine from drinking water”.

Because warm water holds less oxygen, oxygen matters most in a hot tank. Aqueon’s summer guide suggests an aerator “to improve gas exchange at the surface and maximize the dissolved oxygen content in the water”. If fish are in distress, test the water now and contact an aquatic veterinarian. Our guide to aquarium aeration covers aeration in more detail.

Too many fish, or too many at once

Every fish adds waste the filter bacteria have to process. Aqueon’s advice on stocking is to “always err on the side of having fewer fish in your aquarium rather than too many”, and it notes that many fish grow larger after they are bought.

Run a new tank first. Aqueon says a new setup “should be run for a minimum of 2 to 3 days before the first fish are introduced”. That settles the temperature; it does not cycle the tank.
Test before you buy. Aqueon’s first-60-days guide says to “always test water before purchasing new fish” and to wait at least a week between new fish additions.

Sudden pH changes

Aqueon puts stability first. It says “most fish will adapt as long as water quality is good and pH is stable”, and that with fish in the tank you should “never make sudden or drastic changes to pH or any other water parameters”. It says pH-changing products should be avoided or used with extreme care by experienced aquarists, and that liquid adjusters in particular often wear off, so the pH swings back.

Two setup habits Aqueon links to pH trouble:

Topping off instead of changing water. Aqueon lists this among common beginner problems: nitrate and phosphate build up, and pH and alkalinity drop. Regular partial water changes help prevent it. See why aquarium pH keeps dropping for the other causes.
Releasing new fish straight from the bag. Aqueon advises floating the bag and adding small amounts of tank water “every few minutes for half an hour”, and says “the greater the difference” in pH between store and tank, “the more gradually they should be acclimated”. Do not pour store water into your tank.

When the water tests fine

Practical Fishkeeping says that if the water is fine you have to look at other factors, including disease, starvation, long-term stress from unsuitable conditions and old age. Those are outside what a water test or this guide can judge.

Rule out the test itself. Use an in-date kit and test again.
Test your tap water. Aqueon suggests testing tap water too, since some sources are high in nitrate.
Ask for help early. For ammonia that keeps coming back, Aqueon points to your local fish store or an aquarium club. For fish that look unwell, contact an aquatic veterinarian.

Ready to buy?

Our freshwater aquarium test kit roundup compares freshwater test kits, including master kits that cover ammonia, nitrite, nitrate and pH, and our Water Care & Testing section gathers the test kits and conditioners.

Frequently Asked Questions

Q. What is the most common cause of fish death in an aquarium?

Practical Fishkeeping names water quality as the biggest cause. Test ammonia and nitrite first; Aqueon calls zero the only safe level for both.

Q. Why do fish die after a water change?

The new water may not have been treated with a conditioner, or its temperature may not have matched the tank. Size matters too: Aqueon advises avoiding changes of more than 25%, because “larger water changes can stress your fish”.

Q. How many fish deaths are normal?

Practical Fishkeeping estimates that a tank of 50 assorted small fish “may lose a dozen a year”. It says to worry when several fish, or different species, die at once.

Q. How long should I wait before adding new fish after one dies?

Practical Fishkeeping says “leaving it for a full seven days is best”, and only if the water tests well and the other fish look healthy; then add a few fish a week. The magazine adds that “it can be good practice to quarantine new additions first”. If more die in that week, there is an underlying problem to find first.

Q. What should I do with a dead fish?

Do not flush it. Practical Fishkeeping warns of a disease risk to native species through the waterways. Seal it in a bag for the trash, or bury it.

Q. Is my fish dead or just hiding?

Check before you net it. Practical Fishkeeping notes that some catfish wedge themselves into tight spaces and look as if they have died.

Sources (19)

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.