Why are my fish dying? A step-by-step checklist

AquaLog guide · Published 6 October 2026

What should you do first?

1. Remove dead fish straight away. A body rotting in the tank adds ammonia.
2. Test ammonia, nitrite, nitrate, pH and temperature before you change anything.
3. Change 25–50% of the water if ammonia or nitrite is above zero, with dechlorinated water at the same temperature. Stop feeding for a day or two, then feed sparingly until both read zero again.
4. Check the equipment. Is the filter flowing? Does the thermometer agree with the heater setting?
5. Isolate any fish that look sick in a separate tank of clean, dechlorinated water at the same temperature, if you have one. It protects the others and lets you watch the sick fish closely.
6. Write down what you see and what has changed recently. It speeds up the steps below, and anyone helping you will want to know.

Step 1: Is the water safe?

In a tank less than two months old, ammonia or nitrite above zero has a likely answer: the filter bacteria haven't caught up with the fish yet.

Step 2: What changed in the last two weeks?

• New fish, plants or decor. Newcomers can carry disease. See step 4.
• A water change with untreated tap water. The Merck Veterinary Manual notes that chlorine can kill fish suddenly, with no visible signs.
• A big clean. Replacing filter media, or rinsing it under the tap, strips out the bacteria that deal with ammonia.
• A new medicine or water treatment. Some medicines, especially antibiotics, harm filter bacteria. Merck lists copper as toxic to fish above low levels, and our care guide warns against copper medication for cherry shrimp.
• Insect sprays or other chemicals used near the tank. Some household insecticides contain pyrethroids, which are highly toxic to fish.
• A power cut. The filter and heater stop, oxygen falls and the filter bacteria go without the oxygen they need.

Step 3: Is the temperature right and steady?

The Merck Veterinary Manual lists temperature swings, usually from a power or thermostat failure, as a cause of sudden deaths and of infections that follow. Check the heater against a separate thermometer, then check each species' range. Bettas need 24–30 °C (75–86 °F), and our care guide lists chilling in unheated bowls among their common problems. White cloud mountain minnows have a range of 16–24 °C (61–75 °F) and are prone to ich if kept too warm. In hot summers, see fish that tolerate warm water and the cooling tips in why fish gasp at the surface.

Step 4: Are only new fish dying?

Deaths in the first days or weeks after purchase, while your established fish stay healthy, point to the newcomers rather than the tank. Some species travel badly. Our care guide lists shipping stress deaths and starvation in algae-free tanks for otocinclus, poor stock quality for neon tetras, and inbred weak stock for guppies.

• Buy from a shop whose tanks are clean and whose other fish look healthy, as the Merck Veterinary Manual advises.
• Match the temperature and acclimatise new fish slowly rather than tipping them straight in.
• Quarantine newcomers in a separate tank. UF/IFAS specialists advise quarantining all new animals and plants, and the Merck Veterinary Manual says valuable pet fish should be quarantined for at least 30 to 60 days before they join the others.
• Add a few fish at a time so the filter keeps up.

Step 5: Are you feeding too much?

Overfeeding is one of the main causes of high ammonia: fish produce more waste the more they eat, and uneaten food rots into more. Give only what the fish finish in a couple of minutes, and remove anything left on the bottom. See how often to feed fish for the signs of overfeeding.

Step 6: Is the tank overstocked?

Too many fish, or fish too big for the tank, make more waste than the filter and your water changes can handle. Inch-per-gallon rules are a poor guide, because they count length only and leave out body bulk and adult size. Check each species' minimum tank and adult size in the care guide, or enter your tank and fish in the stocking and tank-mate checker, which compares them with our care data. The aquarium volume calculator gives your tank's real volume. A common pleco, for example, grows to 45 cm (18 in) and needs at least 700 litres (185 US gal).

Step 7: Is a fish being bullied?

Look for torn fins, a fish hiding all day, or one missing at feeding time. Some species are hard on others: our care guide lists fin nipping stress in tank mates as a common problem with tiger barbs, which need a group of at least 8, and rates the betta as aggressive. Watch the tank for a few minutes at feeding time, when chasing is easy to spot, and separate a fish that won't settle. See how to choose compatible tank mates.

Step 8: Do the fish show signs of disease?

If the water is good and nothing has changed, look closely at the fish. Signs of illness listed by the Merck Veterinary Manual include:

• lethargy, not eating, or unusually slow or fast breathing
• spots, ulcers (open sores), or puffy or fuzzy growths
• loss of colour, blotches, or scales standing out from the body
• swelling, bloating or weight loss
• floating, drifting or swimming erratically

1. Move sick fish to a separate tank with clean, dechlorinated water at the same temperature, if you have one.
2. Keep testing and changing water in the main tank.
3. Don't guess with medicines. Several diseases look alike, and the wrong treatment can harm fish, shrimp or filter bacteria.
4. Get help from a vet who treats fish, or an experienced aquarist. Some conditions can only be confirmed with laboratory tests.

Wash your hands after working in any aquarium. Some fish bacteria, such as Mycobacterium, can infect people, and the Merck Veterinary Manual suggests that people with weakened immune systems should not handle aquariums directly.

What if nothing explains it?

Small fish have short lives. Our care guide gives 1–3 years for guppies and 2–4 years for platies, so an occasional loss in an older group can simply be age. A steady run of deaths is not normal, though. Keep testing, keep to the maintenance checklist and ask for help early.

When should you get help?

• Fish keep dying although ammonia and nitrite read zero and the temperature is right.
• Fish show spots, sores, fuzzy growths, bloating or scales standing out.
• Several fish fall ill at once, or illness spreads after new fish arrive.
• You are thinking of using a medicine. Get a diagnosis first, then follow the product label.

Contact a vet who treats fish, or an experienced aquarist, and bring your test results, the tank's recent history and a clear photo or video.

Patterns are easier to see written down. AquaLog keeps each tank's fish records, water tests and water changes in one place, so you can check what changed before the losses began.