21 Reasons Why Aquarium Fish Die

So your fish died last night and you don't know why. Most aquarium deaths trace back to water quality: ammonia, nitrite, temperature or pH. The 21 reasons below cover the common causes.

Think about anything you did differently in the last few days. A new fish, new food, a big water change or a spray used nearby is often the clue.

I start with the obvious things and then move on to the not-so-obvious.

Poor or no filtration: Too much ammonia or nitrites

Ammonia is the main waste fish produce. University of Florida researchers note in Ammonia in Aquatic Systems that it is colorless and odorless even at toxic levels, so testing is the only way to detect it.

Make sure your filter is sized for your tank. Rinse mechanical media regularly, but rinse biological media gently in old tank water so you keep the beneficial bacteria. Test for ammonia and nitrite at least weekly; any ammonia reading means something is wrong.

Sudden change in water temperature

A heater can fail or the power can go out, and the water temperature drops quickly. A heater that sticks on can overheat the tank just as fast.

A sudden drop stresses tropical fish. A sudden rise lowers the oxygen the water can hold. Fish can't regulate their own body temperature, so the tank has to do it for them.

Keep a spare heater on hand and use a separate thermometer. Don't rely on the heater's built-in dial.

Sudden change in water pH

New fish dumped straight from the bag can die within days if the store's water was very different from yours. Acclimate new fish slowly to your tank's temperature and water chemistry.

Ask the store what pH and hardness their tanks run at, so you know whether the fish are a good fit for your water.

Untreated tap water

City tap water is treated with chlorine or chloramine to make it safe for people. Both are toxic to fish and to the bacteria in your filter.

Use a water conditioner that removes chlorine and chloramine every time you add tap water. When you rinse filter media, use old tank water, not tap water.

No aquarium heater

Some coldwater fish, like goldfish, don't need a heater. Most tropical fish need stable water in the mid to upper 70s °F, so check the range for each species you keep.

Lack of oxygen in the water

Warmer water holds less oxygen. A stuck heater, direct sunlight or a heat wave can all leave fish gasping at the surface.

Labyrinth fish like bettas can gulp air for a while, but most fish can't. Keep water moving with your filter, an air pump or a powerhead so the surface ripples; that surface movement is where oxygen enters the water.

Didn't cycle the new fish tank long enough

Every new tank needs a nitrogen cycle. Bacteria in the filter convert ammonia to nitrite, then nitrite to much less toxic nitrate.

UF/IFAS notes a new biofilter takes about six to eight weeks to establish. Adding fish before then causes "new tank syndrome." Test until ammonia and nitrite both read zero before adding fish.

Overfeeding the fish

Feed only what your fish eat in about two minutes. Uneaten food rots and releases ammonia, and overfed fish can develop health problems.

Once or twice a day is enough for most adult fish. Grazing herbivores may do better with small, frequent feedings, so check your species' needs.

Fish tank is too small

Less water means waste builds up faster. Bigger tanks are more stable and easier to keep clean, as long as you don't overcrowd them.

Some fish need room to swim or places to hide, and they get stressed in a small tank. Research adult size, schooling needs and temperament before you buy.

Unsuitable fish food

Low-quality food with lots of filler doesn't meet your fishes' needs. Choose a reputable food made for your fishes' diet type.

Carnivores do well with variety, such as frozen bloodworms or brine shrimp. Herbivores and omnivores need plant-based foods like spirulina. Look up what your species eats in the wild.

Disease and parasites

Watch for fish hanging at the bottom or top, swimming oddly, or rubbing against decorations. White specks like grains of salt suggest ich, which UF/IFAS says should be treated as soon as it is spotted. Ragged, fraying fins suggest fin rot.

A swollen body with scales sticking out (often called dropsy) is a sign of serious internal illness and is hard to treat.

Set up a quarantine tank so you can separate sick fish. Treat based on the actual diagnosis rather than guessing, and ask an aquatic veterinarian or experienced fish store when you're unsure. Remove carbon from the filter during medication.

Improper water change

Never remove the fish and replace 100% of the water. The sudden change in temperature and chemistry is a major shock, and you lose much of your beneficial bacteria.

Partial changes of about 25% are a safer routine. Match the new water's temperature to the tank and treat it with dechlorinator before adding it.

Waiting too long between water changes

Your filter turns ammonia and nitrite into nitrate, but nitrate keeps building up. High nitrate weakens fish over time. Live plants absorb some, but water changes are the main way to remove it.

The nitrogen cycle also uses up the water's buffering capacity (alkalinity). When it runs out, pH can crash suddenly, which UF/IFAS calls "old tank syndrome."

Plan on partial water changes every week or two, more often in heavily stocked tanks.

Fish already diseased or sick

Fish can come home already sick. Stores hold many fish from many sources, and some systems share water between tanks, so disease can spread quickly.

Buy from a store with clean tanks and healthy-looking fish. UF/IFAS recommends quarantining new fish for at least 30 days, since they can carry parasites without showing signs.

Using detergents to clean a fish tank

Never use bleach or dish soap on a running tank or its decorations. Residue can kill fish quickly.

For hard water stains on an empty tank, plain white vinegar works. Rinse everything thoroughly with plain water afterward, and never use scouring cleaners or chemical scrubs.

Overcrowding your fish tank

Too many fish means more waste, more ammonia spikes and less oxygen per fish. That adds up to stress and disease.

How many fish a tank can hold depends on the species, their adult size and the tank's surface area. Goldfish produce a lot of waste. Male bettas fight each other. Research adult size and stocking needs for each species.

A wide tank has more surface area than a tall, narrow one of the same volume. More surface means more oxygen exchange, so a wide tank can support more fish.

Using air fresheners

Aerosol sprays can settle on the water's surface. To be safe, cover the tank or spray in another room, and keep plug-ins away from the aquarium.

Using flea spray around the fish tank

Many flea, ant and roach sprays contain insecticides such as pyrethroids that are highly toxic to fish. Spray outside or in another room, cover the tank tightly, and turn off air pumps that pull in room air while you treat.

Noise pollution

Fish sense vibration and pressure changes through their lateral line. Loud speakers next to the tank or tapping on the glass can stress them.

Using hand soap or sanitizer

Hand sanitizer, lotion, perfume, sunscreen and insect repellent can all be toxic to fish. Rinse your hands well with plain water before reaching into the tank, or use aquarium-only tools.

Excessive cigarette or cigar smoke

Nicotine and other smoke residues can dissolve into tank water, especially when an air pump draws in smoky air. Smoke outdoors or keep the tank in a well-ventilated room with a cover.

Final Thoughts:

Test your water regularly for ammonia, nitrite and nitrate, and keep an eye on temperature. Those checks catch most problems before fish die.

If your water tests fine and fish still die, think about recent changes: sprays, new food, new fish, or hands in the tank. That's usually your first clue.

Let me know below if you're guilty of any of the above.

When it comes to fishkeeping, what is your single biggest challenge or frustration? Leave a comment below.

Sources

• Ammonia in Aquatic Systems, University of Florida IFAS Extension
• Ichthyophthirius multifiliis (White Spot) Infections in Fish, University of Florida IFAS Extension