Diagnosis
Why do my fish keep dying?
If the tank is under two months old, one cause accounts for the overwhelming majority of these losses — and it is a water chemistry problem, not an illness.
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In a tank less than about two months old, it is almost certainly ammonia poisoning because the tank was never cycled. Test ammonia and nitrite. If either is above zero, that is your answer and no other diagnosis is needed — change 25 to 50% of the water today and keep changing it until both hold at zero.
This is the most common question in fishkeeping and it usually arrives after two or three losses, when a pattern has become undeniable. The good news is that the answer is nearly always the same, it is not your fault in any meaningful sense, and it is fixable.
Confirm it in ten minutes
Do not diagnose from symptoms. Ammonia poisoning looks like several diseases, and treating a chemistry problem with medication makes it worse by killing the bacteria you need.
- Test ammonia. Anything above 0 ppm in a tank with fish is a problem.
- Test nitrite. Same standard. Nitrite is the second poisoning, and it arrives just as ammonia starts falling.
- If either is above zero, stop here. That is the cause. Go to the protocol below.
- If both read zero, and your test kit is not out of date, then move on to the less common causes further down.
What is happening
A new filter is sterile. It contains none of the bacteria that convert ammonia to nitrite and nitrite to nitrate, so on day one it processes no ammonia at all. Fish produce ammonia continuously, and in that tank it simply accumulates.
Ammonia burns gill tissue well below 1 ppm, and its toxicity rises with pH and temperature — which describes a tropical tank precisely. Damaged gills cannot take up oxygen, which is why fish gasp at the surface in well-oxygenated water.
Then it gets worse before it gets better. Around week two the first bacteria establish and ammonia begins falling — into nitrite, which is also toxic. Keepers who see ammonia dropping and relax often lose fish to the nitrite spike that follows. Two poisonings, in sequence.
What to do today
- Change 25 to 50% of the water now, dechlorinated and temperature-matched. This is the treatment.
- Dose an ammonia-detoxifying conditioner for the full tank volume. Seachem states Prime binds ammonia, nitrite and nitrate for up to 48 hours. It buys time; it does not remove anything.
- Stop feeding for two days, then feed a very small amount once a day.
- Check for a dead fish or rotting plant behind the decor.
- Add bacteria — bottled, or media from an established tank.
- Do not change the filter media and do not clean the filter thoroughly.
- Do not medicate, do not add salt as a first response, and do not add more fish.
- Test daily and repeat the change whenever anything is detectable. Weeks, not days.
Detailed version on how to lower ammonia fast, and the full picture on new tank syndrome.
Quick picks
The two purchases that end this
| # | Product | Best for | Price |
|---|---|---|---|
| 1 | No image API Freshwater Master Test Kit Turns an unexplained series of deaths into a diagnosis in ten minutes. | The first thing to buy, today | |
| 2 | No image Seachem Prime Removes chlorine from the water changes, and detoxifies ammonia between them. | What keeps the survivors alive while the tank catches up |
Rows link to the full write-up further down this page. Prices come from the live price layer only; where there is no live figure the button reads “Check price” rather than showing a remembered number.
If ammonia and nitrite are genuinely zero
Then work through these, roughly in order of how often they turn out to be the answer.
| Cause | What you would see | What to do |
|---|---|---|
| Temperature swings | Tank next to a window, a radiator, or an unheated room. Lethargy, then losses without other signs. | Check with a thermometer at the far end from the heater, not the heater dial. See where to place a filter and heater |
| Chlorine or chloramine | Deaths shortly after a water change | You missed the conditioner, or under-dosed it. Chloramine does not gas off from standing water |
| High nitrate from neglect | An older tank, no recent water changes, high nitrate reading | Bring it down over several smaller changes rather than one large one |
| Overstocking | More fish than the tank can support; ammonia appears intermittently | Rehome or upgrade. See how many fish in a 10 gallon tank |
| Incompatible tankmates | Torn fins, chasing, one fish always hiding | Separate them. Aggression kills slowly through stress |
| Disease brought in with new fish | Visible spots, fuzz, ulcers or unusual swimming; losses started after an addition | This is the case where treatment is appropriate. Identify before medicating |
| Wrong species for the tank | A goldfish in a small tropical tank, or a fish that outgrew it | See what size tank do goldfish need |
| Bad batch from the shop | Only the newest fish die; the established ones are fine | Buy elsewhere. Some shops have persistent problems in their systems |
What not to do
- Do not medicate blindly. Most new-tank deaths are chemistry, not infection, and antibacterial medications kill your nitrifying bacteria — making the actual problem worse.
- Do not tear the tank down and start again. You would be throwing away whatever bacterial colony has developed and starting the clock over.
- Do not replace the filter media. Same reason.
- Do not add more fish to replace the ones you lost, until the water has held at zero for a week.
- Do not trust clear water. Ammonia is invisible. Clarity tells you nothing at all.
- Do not accept "it just happens". It does not. Fish do not die for no reason, and in a new tank the reason is nearly always the same one.
Preventing the next one
Once the tank is stable, the routine that keeps it that way is short: weekly 25% water changes, weekly testing, sparing feeding, and adding fish slowly enough that the colony keeps up.
And for any future tank, cycle it before the fish. Four to six weeks with nothing at risk, or under two weeks if you seed the filter from an established tank. It is the difference between a first aquarium that works and one that becomes an expensive lesson: how to cycle a fish tank.
Questions people actually ask
Frequently asked questions
+ Why do my fish keep dying in a new tank?
+ How do I know if it is ammonia or a disease?
+ My water is crystal clear, so why are fish still dying?
+ Should I do a full water change if fish keep dying?
+ Can fish recover from ammonia poisoning?
+ How long until the tank is safe?
Where the numbers came from
Sources
- US EPA, Aquatic Life Criteria for Ammonia — toxicity and its dependence on pH and temperature
- USGS, The Nitrogen Cycle — nitrification as a two-stage bacterial oxidation
- Seachem Prime — product page (dose, what it removes, what it detoxifies, 48-hour binding claim)
- API Freshwater Master Test Kit — Amazon listing (parameters tested, test count)
Read next
Related on Fin & Filter
How to lower ammonia in a fish tank fast
The emergency protocol in order: water change first, conditioner second, stop feeding, and what not to do.
New tank syndrome: what it is and how to fix it
The symptoms, the test results that confirm it, and the emergency protocol for a tank with fish already in it.
How to cycle a fish tank
What cycling actually is, the fishless method start to finish, and the exact test result that means it is done.