The short answer
Most often the water: Practical Fishkeeping, a UK fishkeeping magazine, calls water quality “by far the biggest killer of fish”. Goldfish produce a lot of waste, so ammonia and nitrite can build up fast in a bowl, a crowded or uncycled tank, or behind a filter too small for them. Test ammonia and nitrite first.
For the causes that apply to every species, see our guide to the nine common causes of fish deaths.
Warning
If a goldfish is sick or dying right now, test the water now (ammonia, nitrite, nitrate, pH and temperature) and contact an aquatic veterinarian. This guide covers water, tank and equipment causes only: fix what the test shows, and leave identifying and treating illness to the vet.
If a goldfish is dying or has just died
Practical Fishkeeping’s advice is that “every time you find a dead fish you must test the water”. The magazine says to remove any dead fish, since a body pollutes the water. Then:
Test the water and read the thermometer
A liquid kit such as the API Freshwater Master Test Kit checks “ammonia, pH, high range pH, nitrite and nitrate”, says API, a test-kit maker. Compare your readings: Aqueon, an aquarium equipment maker, says “the only safe ammonia level is zero”, the same for nitrite, and that nitrate “levels above 50 ppm are undesirable in freshwater aquaria”. The goldfish pH and temperature ranges are in the sections below. Confirm the filter and any air pump are running. If the temperature is outside the range, Aqueon 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, since fed fish “produce more waste” (Aqueon’s ammonia guide gives no length of time; ask your aquatic store or veterinarian when to resume). The sources differ on how much to change, and so do Aqueon’s own pages. Its ammonia guide says to “change between 25 to 50% of the tank’s water”; its 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”. The RSPCA, a UK animal welfare charity, says that for readings above zero “you’ll need to change half of the water every day and keep testing until conditions improve”. For ammonia, API’s FAQ says to “simply perform a partial water change if ammonia is high upon testing”. Treat the new water with a conditioner first and match its temperature: Aqueon’s goldfish guide warns that “rapid changes in temperature or water chemistry can be harmful, if not fatal to goldfish”.
Keep testing, and get help if the readings do not fall
If waste levels do not come down over time, the RSPCA says: “Ask your aquatics shop for help with this.” Aqueon’s ammonia guide adds: “Don’t wait until your fish get too sick and die before seeking professional aquarium help.” Our guide to why aquarium ammonia gets high covers the usual sources.
For any goldfish still alive but unwell,
test the water and contact an aquatic veterinarian.
Why goldfish are hard on their water
Aqueon’s goldfish guide explains: “In addition to having relatively high oxygen demands, they grow to be quite large and generate a significant amount of waste, which can be harmful if allowed to accumulate.” Fish waste becomes ammonia; filter bacteria turn it into nitrite, which the RSPCA notes is “also toxic”, and then nitrate. Aqueon’s goldfish guide says “pH is not critical, but ideally should be between 7.0 and 8.4”, and its water-quality guide notes that ammonia “is more toxic at higher temperatures and pH levels above 7.0”.
You cannot see any of this. The RSPCA says “clear water doesn’t always mean healthy water”, and calls kits for ammonia and nitrite “essential”. The feeding row below is Aqueon’s: “feed only what they can consume in 2 to 3 minutes, once or twice a day”.
| Cause | How to check | Setup fix |
|---|---|---|
| Bowl or small tank | Gallons per goldfish | A bigger, filtered tank |
| Filter too small or off | Is it running day and night? | An oversized filter, left on |
| Uncycled tank | Ammonia and nitrite tests | Cycle first (see below) |
| Overfeeding | Food left after 2-3 minutes | Feed less |
| Untreated tap water | Was every refill treated? | Conditioner on all tap water |
| Temperature swings | Thermometer, daily | Steady room; slow changes |
| Low oxygen | Thermometer; surface movement | Aeration; filter outlet at surface |
Bowls and small tanks
Aqueon is blunt: “Goldfish should never be kept in bowls, small aquariums or any unfiltered container!” The RSPCA lists why a bowl fails: “there isn’t enough space for an efficient filter, so waste products can’t be removed” and “the low surface area means that not enough oxygen will be absorbed into the water”.
Goldfish also grow. Practical Fishkeeping’s goldfish FAQ (2005) says they “have the potential to get to at least 15cm/6″ and sometimes double that”. The current sources set minimums differently:
| Source | Minimum water | Scope |
|---|---|---|
| Aqueon (maker) | At least 20 gallons per fish | Adult common, comet, shubunkin |
| Aqueon (maker) | At least 10 gallons per fish | Adult fancy goldfish |
| RSPCA (charity) | At least 60 liters each (about 16 US gallons) | Goldfish, no type split |
The 16-gallon figure is worked out from the RSPCA’s liters (60 ÷ 3.785). The RSPCA also gives a rule that grows with the fish: “Allow 2 litres of water per 0.5cm of goldfish length”. Worked out, its 60 liters fits a goldfish about 15 cm (6 in) long, and a bigger fish needs more. It adds that these figures “should only be used as guidelines”. Our Tanks & Stands section covers tank options.
Filtration and a pump that runs all night
Aqueon’s goldfish guide says “the filter should be slightly over-sized to accommodate high waste production”, and its pet-fish guide says that for messier fish “like goldfish or cichlids”, “buying a slightly oversized model is a good idea”.
The filter also has to keep running. In a Practical Fishkeeping letter, a reader’s daughter switched her pump off every night so she could sleep. Jeremy Gay’s first suspicion was “poor water quality as a result of the pump being turned off”, and he wrote: “All pumps should run for 24 hours per day”. For oxygen, Gay suggests you “keep the filter outlet at the water surface, making ripples, to increase the oxygen content”. Our Filters & Media section covers aquarium filters.
Goldfish that die after a tank clean
Aqueon’s do’s and don’ts warns: “Don’t completely tear down your aquarium to clean it. This will destroy the biological balance with disastrous results.” To clean without restarting the tank:
If you have an internal filter, turn it off, then vacuum the gravel in place
Practical Fishkeeping’s 2005 FAQ gives this order for an internal filter, with a gravel cleaner. For an undergravel filter, it says the gravel holds helpful bacteria, and “if you wash it, you’ll destroy them”.
Rinse the filter sponge or media in the water you siphoned out
The RSPCA says to swish the sponge or media in removed tank water, and to “never wash it under the tap as you’ll kill the beneficial bacteria”. Put the sponge back and switch the filter on again once you finish; see the pump section above.
Change part of the water, not all of it
For routine care, Aqueon’s goldfish guide says to “change 10% of the water weekly or 25% every 2 weeks”; the RSPCA’s page says to “remove ten percent of the water with a siphon” and also calls for “weekly 30% water changes”. The sources differ. See why aquarium water changes matter for more.
Treat every refill
Practical Fishkeeping’s FAQ notes that tap water “contains chlorine (and possibly chloramine, depending on where you live)”. Use a conditioner whose label covers chloramine; API’s FAQ says its Stress Coat “instantly removes chlorine and chloramines from tap water”.
Test ammonia and nitrite afterwards
If either reads above zero, follow the water-change steps above.
New goldfish that die in the first days
A new tank lacks the bacteria that clear ammonia and nitrite, and the sources build them two ways. Aqueon describes cycling with fish: it “begins after you add the first fish”, and its safest way is to “add just a few fish initially”, feed sparingly and test ammonia and nitrite “until they stabilize at zero before adding more fish”. The RSPCA “strongly” advises fishless cycling instead, and in that advice says to “only add fish when there has been a spike in the levels of ammonia and nitrite, followed by a return to zero.” Our guide to why a tank needs cycling explains the process.
- Add a few at a time. Aqueon says to “wait at least a week between new fish additions”.
- Consider a quarantine tank. Practical Fishkeeping says “it can be good practice to quarantine new additions first”.
Temperature and oxygen
The sources give goldfish ranges by type:
| Source | Fancy goldfish | Common, comet, shubunkin |
|---|---|---|
| Aqueon (care guide and summer guide) | 68° to 74°F | 60° to 70°F |
| RSPCA | 10–21°C (about 50–70°F) | Same range, no type split |
For 60° to 70°F, the care guide names comets and shubunkins and the summer guide common goldfish. Aqueon’s other pages are less specific: one gives goldfish “68° to 74° F” with no type split, and another says goldfish “prefer temperatures below 70°F”. The RSPCA’s upper limit, 21°C (about 70°F, worked out from its Celsius range), is lower than the top of Aqueon’s fancy-goldfish range. The sources differ here. The RSPCA warns that “thermostats and heaters can fail, so use a digital thermometer and check the temperature every day”. Our Heaters & Thermometers section covers heaters and thermometers.
Warmth and oxygen are linked. Aqueon’s summer guide notes that “the ability of water to hold dissolved oxygen drops with rising temperature”, and its goldfish guide says “additional aeration is recommended when goldfish are kept at warmer temperatures”. See our guide to aquarium aeration for the equipment.
When the water tests fine
- Test again with a second kit. Jeremy Gay asked the reader: “Did you get a second opinion from another test kit or from someone in an aquatic store?”
- Look for household chemicals near the tank. He lists “perfumes, sprays, scented candles, pet flea treatments” among outside sources that may not show up on a water test, and the RSPCA warns that “cigarette smoke or aerosol sprays can be toxic to fish”.
- Ask a vet. Practical Fishkeeping says that if the water is fine, look at other factors, including disease. That is for an aquatic veterinarian, not a water test.
Ready to buy?
Our freshwater aquarium test kit roundup compares freshwater test kits, and our Water Care & Testing section gathers the test kits and conditioners.
Frequently Asked Questions
Q. Why did my goldfish die overnight?
If a pump or filter was off for the night, see the pump section above. Test the water now; if other fish look unwell, contact an aquatic veterinarian.
Q. How do goldfish act before they die?
Reading behavior is a job for an aquatic veterinarian. If a goldfish looks unwell, test the water now and contact a vet.
Q. Do goldfish prefer long or tall tanks?
The RSPCA says “wide tanks are better than tall tanks as they have a larger surface area for the water volume”, and the surface is where oxygen enters the water.
Q. Do goldfish need a heater?
The RSPCA says a heater “isn’t normally necessary” for goldfish, though “if you are concerned about temperature swings, you could include a heater, set at the lower limit of the range”.
Sources (15)
- Practical Fishkeeping (magazine), Why do our goldfish keep dying?
- Practical Fishkeeping (magazine), Frequently asked questions on goldfish (2005)
- Practical Fishkeeping (magazine), What to do if you find a dead fish in your tank
- Aqueon (maker), Goldfish Care Guide
- Aqueon (maker), Freshwater Aquarium Water Quality
- Aqueon (maker), 15 Common Fish Tank Problems and How to Avoid Them
- Aqueon (maker), How to Detect and Fix Aquarium Ammonia Issues
- Aqueon (maker), Aquarium Do’s and Don’ts
- Aqueon (maker), How to Help Keep Your Pet Fish Alive
- Aqueon (maker), Starting a New Aquarium: What to Expect in the First 60 Days
- Aqueon (maker), Water Temperature and Fish Health
- Aqueon (maker), How to Keep Your Aquarium Cool in Summer Heat
- RSPCA (UK animal welfare charity), Choosing an aquarium for pet fish
- API (maker), Freshwater Master Test Kit
- API (maker), Frequently Asked Questions
