Guide
Why are my fish dying when water parameters are OK?
Ammonia and nitrite are zero - so what else could be killing your fish?
- Zero ammonia and nitrite doesn't mean the water is safe: chlorine, chloramine, low oxygen, temperature swings, and nitrate buildup can all kill fish while your basic tests look fine.
- Disease is the most common hidden killer: internal parasites and bacterial infections often show no obvious symptoms until the fish is already dying - skipping quarantine is the single biggest risk factor.
- Aggression leaves no evidence: a bullied fish can die from stress and starvation with zero visible injuries, and you'll never see it happen if you're not watching at feeding time.
- Test kit accuracy matters: liquid reagents expire, strip tests are unreliable, and a large water change right before testing can mask a genuine ammonia spike.
- New fish introductions reset the clock on everything: pathogens, temperature shock, and oxygen crashes are all more likely in the 72 hours after adding livestock.
- Isolate before treating: move sick or dying fish to a hospital tank immediately - treating the main tank with medication can crash your cycle and harm healthy fish.
Why "parameters are fine" isn't the whole picture
The aquarium hobby has a habit of reducing water quality to ammonia, nitrite, and nitrate. Those three matter enormously - but they're not the only things in your water that can kill fish. When you test and get zeros across the board and fish are still dying, you haven't ruled out water quality as a cause. You've only ruled out those three compounds.
Before you move on to disease or aggression, take five minutes to pressure-test your test results. Liquid test kits (API Master Test Kit is the standard) are reliable when the reagents are fresh and you follow the instructions exactly. Strip tests are not reliable - the reagents degrade faster and the colour matching is too imprecise for a tank in trouble. Check the expiry dates on your reagents. If the bottles are more than a year old or have been stored somewhere warm, they may be giving you false negatives. See our guide to aquarium water parameters for what accurate testing actually looks like.
Also consider timing. Ammonia spikes can appear and be partially converted within hours. If you test on Sunday morning and your fish died Saturday night, the reading you get may genuinely be zero - because the cycle processed the ammonia already, or because a partial water change diluted it. The absence of ammonia now doesn't prove there was no spike then.
Most people test once, get zeros, and conclude water quality isn't the problem. One test is a snapshot, not a history. If you're losing fish, test daily for a week - morning and evening if possible - to catch spikes you'd otherwise miss.
Chlorine and chloramine
This one kills fish fast and leaves no trace by the time you go looking. Tap water in most municipal supplies contains either chlorine, chloramine, or both. Chlorine dissipates within 24 hours if you leave water to stand, but chloramine - which is increasingly common because it's more stable - does not. It binds to haemoglobin and destroys gill tissue.
If you added tap water directly to your tank without dechlorinator, even once, that's a plausible cause of death regardless of what your ammonia test says. Standard sodium thiosulphate dechlorinators neutralise chlorine but not chloramine. You need a product that specifically states it neutralises chloramine - Seachem Prime is the most widely used option and also detoxifies low levels of ammonia temporarily. Add it to the new water before it goes in the tank, not after.
Water companies sometimes switch between chlorine and chloramine seasonally or during infrastructure work, without notifying residents. If fish deaths coincide with a water change and everything else checks out, contact your water supplier to ask what's currently in use.
Oxygen levels
Most home test kits don't include dissolved oxygen, so it goes unmeasured. That's a problem, because oxygen depletion can kill an entire tank overnight - and the conditions that cause it are common.
Fish need dissolved oxygen to breathe. Oxygen enters the water at the surface through gas exchange, and that process is driven by surface agitation. A filter that returns water below the surface, a heavily planted tank with CO2 injection at night, a hot summer without an air stone, or a tank that's significantly overstocked - any of these can crash oxygen to lethal levels while ammonia, nitrite, and nitrate all read normal.
The classic symptom is fish gasping at the surface, particularly first thing in the morning when overnight biological activity has consumed oxygen and plant photosynthesis hasn't yet started. But by the time fish are visibly gasping, oxygen is already critically low. If you're losing fish and your surface is calm and glass-like, add an air stone and increase surface movement immediately. It costs nothing and the risk of not doing it is high.
Warm water holds less dissolved oxygen than cold water. In summer, a tank that was perfectly oxygenated at 24°C (75°F) can become oxygen-depleted at 28°C (82°F) with no other change.
Temperature shock and heater failure
Fish are cold-blooded, meaning their metabolism, immune system, and ability to fight disease are all directly tied to water temperature. A sudden shift of more than 2°C (3.6°F) in either direction causes physiological stress significant enough to suppress immune function for days - even if the fish looks outwardly fine. Heater failures are common and often go unnoticed. Heaters can fail in the "on" position, cooking a tank, or in the "off" position, allowing temperature to drop to room temperature overnight.
Water changes are another source of temperature shock. If you're adding water that's 4-5°C cooler or warmer than the tank, you're stressing every fish in there. Use a thermometer on the water change bucket every time - not approximately the same temperature, actually the same.
Run two thermometers in any tank where you're having unexplained deaths - one in the tank, one to test your change water. Heater thermostats drift over time and may no longer reflect the actual temperature shown on the display dial.
Disease and parasites
This is the most common cause of fish deaths when water quality genuinely is good, and it's also the one most people investigate last. Internal parasites, bacterial infections, and viral diseases frequently kill fish with minimal external signs. A fish that's "just hiding more than usual" or "not as interested in food" may be days away from death.
The risk multiplies every time you add new fish. A fish that looks healthy at the fish store may be carrying Ichthyophthirius multifiliis (ich), Pleistophora hyphessobryconis (neon tetra disease), internal worms, or bacterial infections - all in an incubating state that shows no visible signs for days or weeks. Once that fish goes into your main tank, it's exposed to whatever minor stressors exist there, and the pathogen can spread before you've identified it.
Quarantine is the only reliable prevention. A separate tank - even a simple 40-litre (10 US gallon) bare-bottom setup with a heater, filter, and lid - running for four weeks will reveal most diseases before they reach your display tank. Most fishkeepers who skip quarantine will eventually lose a tank to an introduced pathogen. It's not a matter of if.
The fish trade has little incentive to tell you this, but most fish stores hold fish at stocking densities and stress levels that suppress immune function. Fish at a store that "looks fine" is not a guarantee - it's a fish managing its stress load in a controlled environment. Your tank, with different water chemistry, different tankmates, and a new transport experience, may be the trigger that tips it over.
Aggression and chronic stress
Stress kills fish. Not quickly, not dramatically - but consistently. A fish that's being bullied, out-competed for food, or prevented from reaching certain areas of the tank will experience elevated cortisol, suppressed immune function, and gradual physical deterioration. The fish often shows no wounds. It just gets thinner, hides more, and eventually dies.
The aggressor isn't always obvious. Many species that are sold as "community fish" are territorial in practice, especially in smaller tanks or when stocked at the wrong ratio. Male guppies kept without enough females will harass each other. Cichlids will claim territories and block access to food. Even normally peaceful species like pearl gouramis will bully each other if you keep two males in a tank under 120 litres (30 US gallons).
Watch your tank during feeding. Every fish should get food without being chased off. If one fish is eating while others scatter, you have a stocking problem. Check species compatibility carefully before adding anything - our free stocking calculator can flag obvious mismatches, and the guide to best fish for beginners covers species that genuinely work in community setups.
Nitrate - the slow killer most people ignore
Nitrate doesn't kill fish the way ammonia does - it's a slow, chronic toxin rather than an acute one. But at sustained levels above 40 ppm, nitrate stresses fish, suppresses immune function, and makes them vulnerable to diseases they'd otherwise fight off. Some sensitive species - discus, many wild-caught fish, and most invertebrates - show stress at levels above 20 ppm.
If your tank has good ammonia and nitrite readings but elevated nitrate, your water change schedule may not be keeping up with your bioload. A heavily stocked or heavily fed tank in a 200-litre system can accumulate nitrate faster than a weekly 30% water change removes it. Test specifically for nitrate if you haven't been tracking it - it's easy to overlook because it doesn't cause the visible, acute deaths that ammonia does. Our guide on how often to do water changes covers how to calibrate your schedule to your actual bioload.
Sensitive species like discus and many tetras show immune suppression at levels above 20 ppm - well before any obvious symptoms appear.
A new tank that isn't fully cycled
If your tank is less than six to eight weeks old and you never did a fishless cycle, your biological filtration may not be established enough to process the ammonia your fish are producing - even if a single test reads zero. Ammonia can spike, get processed partially, spike again, and produce readings that look acceptable on a schedule that doesn't match when you're testing.
A fully cycled tank shows zero ammonia, zero nitrite, and measurable nitrate - consistently, including 24 hours after feeding. If you're seeing any nitrite at all, your cycle isn't complete. See the guide on how to cycle a fish tank and the explanation of the nitrogen cycle for what you should actually be seeing at each stage.
Adding fish before the cycle is complete - or adding too many fish at once after cycling - is the single most common cause of unexplained fish deaths in new tanks. "Cycling with fish" is possible but requires very small bioloads, daily testing, and frequent water changes. Most beginners underestimate how long it takes.
What to do right now
If you're losing fish today, work through this in order:
1. Isolate any fish showing symptoms - lethargy, clamped fins, gasping, loss of colour, visible lesions - into a separate hospital tank. Don't treat the main tank blind.
2. Test ammonia, nitrite, nitrate, and pH with fresh liquid reagents. If you don't have them, go to a fish store and ask them to test your water - take a 500ml sample in a clean bottle.
3. Check temperature with a separate thermometer, not just the heater dial. Check the water you'd use for a water change at the same time.
4. Increase surface agitation immediately if you haven't already. Add an air stone if you have one.
5. Do a 25-30% water change with dechlorinated, temperature-matched water. This dilutes any toxins and buys time while you investigate.
6. Watch the tank for 20 minutes during and after feeding to look for aggression you might have missed.
Track every test result and every death with dates and timestamps. App-aquatic lets you log water parameters and set reminders so you can spot patterns - a series of deaths always following a water change, for example, points directly to a dechlorinator or temperature problem.
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's happening?
Zero ammonia and nitrite rules out acute nitrogen toxicity but not much else. The most likely causes are disease introduced by new fish, chloramine in tap water that wasn't neutralised, chronic nitrate stress (test specifically for nitrate if you haven't), oxygen depletion, or aggression. Work through each systematically - start with a full parameter test including nitrate and pH, then check temperature, then observe for aggression during feeding.
Could a water change have killed my fish?
Yes, in several ways. Untreated tap water containing chlorine or chloramine will damage gill tissue within minutes. Water that's significantly colder or warmer than the tank causes temperature shock. A very large water change (over 50%) in an established tank can also cause a pH or hardness shift large enough to stress fish. Always dechlorinate, always match temperature, and keep changes to 25-30% for routine maintenance.
How do I know if my fish died from disease or poor water quality?
Water quality deaths tend to affect multiple fish simultaneously or in quick succession, and often follow a water change or new addition. Disease deaths are more likely to progress through the tank gradually, starting with one or two fish and spreading over days or weeks. Visible symptoms like white spots, bloating, fin damage, or lesions point to disease. If fish are dying with no visible symptoms, test water first - but if water checks out, treat for disease as the likely cause.
Is it normal for fish to die after adding new fish to the tank?
It's common, but not normal in the sense of being acceptable or inevitable. Deaths after adding new fish are almost always caused by one of three things: an introduced pathogen from the new fish, aggression from existing fish defending territory, or a temporary ammonia spike from the increased bioload in a tank that was already near its biological capacity. Quarantine new fish for four weeks, add fish gradually, and recheck your ammonia for a week after
More guides · Suddenly losing fish · Why did my fish die suddenly? · Quarantine new fish