Why Are My Fish Dying? 10 Real Causes and How to Fix Each
Fish almost never die for no reason. Work through these ten causes in order of likelihood, starting with the one that is responsible for the majority of home aquarium deaths in India.
When fish die, most people assume disease and reach for medication. In reality, the large majority of home aquarium deaths in India come down to water chemistry, temperature, or oxygen, and medicating a water quality problem makes it worse. Work through this list in order. The first three causes account for most of what you will find.
Start Here: What the Pattern Tells You
| Pattern | Most likely cause |
|---|---|
| All fish died within hours | Chlorine, chemical contamination, temperature crash, or oxygen loss |
| New fish died within 3 days, old fish fine | Shipping stress, no acclimatisation, or introduced disease |
| Fish dying one every few days | Ammonia or nitrite from an uncycled tank |
| Only the biggest or smallest fish dying | Aggression or bullying, or being outcompeted for food |
| Fish gasping at the surface before dying | Low oxygen, high ammonia, or gill damage |
| White spots, fuzz, or ragged fins before dying | Disease, usually triggered by a temperature or stress event |
| Deaths started after a water change | Chlorine, temperature mismatch, or too large a change |
1. New Tank Syndrome (Ammonia and Nitrite)
This is the number one killer, full stop. A brand new aquarium has no bacteria colony to convert fish waste. Ammonia builds up, burns gills, and fish die one by one over the first three to six weeks while the water still looks perfectly clear.
Signs: gasping at the surface, red or inflamed gills, lethargy, fish sitting on the bottom, deaths spread over days in a tank set up less than two months ago.
- Test ammonia and nitrite today. Any reading above 0 confirms it.
- Do a 30-50% water change immediately with dechlorinated, temperature matched water.
- Repeat 25-30% daily until both read zero.
- Stop feeding for 48 hours, then feed a quarter portion every other day.
- Add a bottled bacteria starter (Seachem Stability, API Quick Start, Tetra SafeStart) daily as directed.
- Do not add any more fish until ammonia and nitrite hold at zero for a week.
- Do not clean the filter during this period.
⚠️The correct way to avoid this is a fishless cycle before you buy anything: run the tank for 4-6 weeks with a source of ammonia until it can process it fully. Everyone who skips this pays for it in dead fish.
2. Chlorine or Chloramine
Untreated tap water kills fast, often within hours, and typically takes the whole tank. Indian municipal water uses chloramine, which does not evaporate off no matter how long the water stands. If you did a water change without conditioner and fish died the same day, this is your answer.
Fix: always add water conditioner to new water before it enters the tank. If you suspect exposure right now, dose the whole tank with Seachem Prime at the emergency rate on the bottle and add heavy aeration.
3. Overfeeding
Uneaten food rots, which produces ammonia, which does the killing. Overfeeding also causes bloating, constipation and swim bladder problems in fancy goldfish and bettas. Almost every beginner overfeeds.
Fix: feed once a day, only what disappears in two minutes, and net out anything left. One fasting day a week is good for most species.
4. Low Oxygen
Fish hanging at the surface with rapid gill movement, especially in the early morning or on hot afternoons. Causes: high temperature, overstocking, no surface agitation, a stopped filter, or a bacterial bloom consuming oxygen.
Fix: add an air stone immediately, increase surface movement, reduce stocking, and cool the tank if it is above 30°C. Oxygen enters water only at the surface, so a still surface is a suffocating tank.
5. Temperature Shock
A change of more than 2-3°C in a short time causes immune suppression, and white spot (ich) outbreaks usually follow within a week. Common causes in India: cold tap water in winter water changes, a failed heater, an unheated tank in a north Indian winter, or a tank cooking in afternoon sun.
Fix: get a digital thermometer, match water change temperature, and use a heater sized at roughly 1 watt per litre where winters are cold.
6. Overstocking
More fish means more waste, less oxygen, and more aggression. The classic Indian version is 8 goldfish in a 1 foot tank, or a common pleco and an Oscar sharing 60 litres. Goldfish need 40-75 litres each. Oscars need a 4 foot tank minimum.
Fix: rehome, upgrade, or split the stock. No amount of filtration compensates for a tank that is too small for the fish in it.
7. Incompatible Tank Mates
Fin nipping, chasing and stress kill slowly and are easy to miss because it happens at night or while you are out. Common mismatches: male bettas with guppies or any long finned fish, tiger barbs with angelfish, goldfish with tropical fish (different temperature needs), and any small fish with an Oscar or flowerhorn, which will simply eat them.
Fix: check compatibility before buying, provide hiding places and sight breaks, keep schooling fish in groups of six or more, and separate the aggressor if needed.
8. Disease
| Symptom | Likely disease | Treatment |
|---|---|---|
| White salt-like spots on body and fins | Ich (white spot) | Raise temperature to 30°C gradually, add aquarium salt, or use an ich medication for 7-10 days |
| Cotton wool tufts on body or mouth | Fungus / columnaris | Antifungal or antibacterial treatment; improve water quality |
| Ragged, receding fins | Fin rot | Water changes plus antibacterial treatment; almost always caused by poor water |
| Body swollen, scales sticking out | Dropsy | Usually a symptom of organ failure; prognosis is poor, isolate the fish |
| Swimming sideways or floating | Swim bladder disorder | Fast 2-3 days, then feed a deshelled boiled pea; common in fancy goldfish and bettas |
| Gold or rust coloured dust on body | Velvet | Darken the tank and treat with a copper-based or velvet-specific medication |
💡Before medicating anything, test your water. Fin rot, fungus and ich are almost always secondary infections that took hold because ammonia, temperature or stress weakened the fish first. Treating the symptom while the cause is still running gets you nowhere.
9. Poor Acclimatisation
Dropping a bag of fish straight into a tank with different temperature, pH and hardness is a shock most fish survive but some do not, and the survivors are stressed enough to fall sick within days. Always float the bag for 20 minutes, then add tank water to the bag in small amounts over 30-45 minutes before netting the fish across. Do not pour shop water into your tank.
10. Contamination From Outside the Tank
- Hands washed with soap or sanitiser going into the tank. Rinse thoroughly with plain water first.
- Mosquito repellent spray, room freshener, incense smoke, or insecticide near the tank. Aerosols get pulled in by the air pump.
- Paint fumes or fresh varnish in the room.
- A bucket previously used for detergent.
- Copper or brass items and untreated metal decor.
- Cooking oil vapour from a nearby kitchen forming a film on the surface, which blocks gas exchange.
The Emergency Protocol
If fish are dying right now and you do not know why, do this in order:
- Remove any dead fish immediately.
- Add an air stone or increase surface agitation.
- Do a 30% water change with dechlorinated, temperature matched water.
- Test ammonia, nitrite, nitrate and temperature.
- Stop feeding for 24-48 hours.
- Do not add medication until you have a test result. Do not clean the filter.
- Repeat 25% water changes daily until parameters are stable.