Why Are My Fish Dying? What to Check First

Do this first

1. Test the water. Test ammonia, nitrite, nitrate and pH, and take the temperature. A veterinary manual says many fish disorders come from stress, poor water quality, overcrowding and failure to quarantine.
2. Test your tap too. Some tap water contains ammonia, which changes what a water change will do.
3. Note the tank’s age. A new tank and an old tank can read the same and need opposite fixes.
4. Remove any dead fish. This is our advice, because a dead fish adds waste and can carry disease.

No test kit? A veterinary manual says a veterinarian or pet store may test a sample for you. Our water guide explains the readings.

New tank or old tank?

	New tank	Old tank
Name	New tank syndrome	Old tank syndrome
When	Usually 1 to 3 weeks after setup	Long-running tank
Ammonia	High	Very high, often above 10 mg/L
pH	Usually normal	Very low, below 6.0
Cause	Too few bacteria in the filter	A pH crash kills the bacteria
Water change?	Yes, partial	Not a simple one

New tank: the most common cause

A fish veterinarian calls new tank syndrome the most common cause of death in new tanks. The bacteria have not developed yet, so ammonia and nitrite build up.

That veterinarian says to do a partial water change straight away if any of these show:

• ammonia above 0.1 mg/L
• nitrite above 0 mg/L
• nitrate above 20 mg/L

Keep to these limits when you do:

• Do not replace more than 50% at once, that veterinarian says. A veterinary manual is firmer about a higher reading, saying to change at least 50% once total ammonia reaches 2 mg/L. So about half is where the two meet at a high reading.
• Change less if the waters differ. If tank and tap pH differ by more than 0.5, make smaller, gradual changes.
• Treat and match the new water. Use a conditioner, and match the temperature.
• Keep testing, daily or every other day.

Then finish the job with our cycling guide.

Sources differ on feeding. A university guide says to reduce or stop feeding for 1 to 3 days when ammonia is present, except for fry. A fish veterinarian says to avoid fasting fish to speed up the cycle. Our advice: feed lightly, and remove anything uneaten.

Old tank: be careful

This is the one case where a water change can be dangerous. A veterinary manual explains that when old water is never removed, only topped up, acids use up the water’s buffering. Eventually pH crashes and the filter’s bacteria die.

The manual says simple water changes are not recommended here, because a rise in pH can turn the ammonia present toxic and kill the fish. Instead it says to move the fish to a separate holding tank, break down and clean the aquarium, and set it up again as a new system.

This is serious, so ask a fish vet or an experienced keeper to talk it through if you can.

If the water tests look fine

What changed recently?	Possible cause	What to check
New fish arrived	Disease brought in	Quarantine newcomers from now on
Power cut, or filter stopped	Low oxygen	Moving water and aeration
Cleaned nearby, or hands in the tank	Outside toxins	Soap, lotion, perfume and cleaning sprays
Heater failed or room got cold	Temperature	A separate thermometer
Added medicine	Filter bacteria harmed	Test ammonia more often
More fish, or more food	Waste, and aggression	Stocking and compatibility

A university guide also warns that fish may become sick weeks after a water problem has been corrected. So one good test today does not prove the water was good last week.

Signs that point to illness

A veterinary manual lists these as common signs of illness:

• lethargy, or not eating
• slow or rapid breathing
• loss of color, spots, or open sores
• scales standing up, swelling or bloating
• floating, drifting or erratic swimming
• puffy or fuzzy growths

More complicated diseases need veterinary treatment, the manual says. If fish keep dying and you cannot find the cause, a fish vet is the right next step. We give no medication doses here, and our disclaimer explains why.

Stop it happening again

• Cycle before stocking, and add fish a few at a time.
• Remove old water when you change it. Do not only top up.
• Test on a schedule, and whenever fish look different.
• Quarantine new arrivals.
• Do not overfeed. A university guide calls it a major cause of high ammonia.