The death of a fish in the aquarium can stir up many emotions including sadness, anger, and frustration whether it was prized or not. Few fish die of old age and many fish die shortly after they’ve been added to the tank. But why? Many questions and theories can go through your mind, so here are some answers to give you closure with a list of the top causes of fish death in the aquarium.

Water Quality

Poor water quality is the single biggest killer of aquarium fish. Water can be crystal clear but deadly at the same time, so even though the tank environment may look clean, healthy, and inviting, toxins may poison the water and end in your fish’s untimely death.

Testing for ammonia before introducing new fish reduces the chance of fish death
API Ammonia Test Kit for Fresh and Salt Water

The worst toxin in the aquarium is actually produced by the fish themselves - ammonia - excreted via the gills through respiration or urea. Although they produce ammonia, they are not evolved to deal with it. In nature ammonia levels are diluted by the large water volume the fish live in or swept away by the current. However, in new aquariums where beneficial bacteria colonies aren’t present or functioning, it is common for ammonia to build up, raising a fish's internal pH and killing it.

When present, ammonia is converted by certain bacteria into nitrite, which is also very toxic and also responsible for fish deaths. Another form of bacteria needs to be present to convert that nitrite into less toxic nitrate, which completes the Nitrogen cycle in the tank, classing the aquarium as "cycled". However, if a fish dies, and tests show that nitrite is present, it is usually the cause commonly referred to as New Tank Syndrome. So, be sure your tank is cycled and ready to hold new fish.


Disease can cause fish death and should be treated with medication

If water quality, oxygen, and temperature are all as they should be at the time of death, the fish may have had a disease. Disease is probably the second biggest killer of aquarium fish, after water quality. There are dozens of common diseases that fish can succumb to and can appear from nowhere as many are not visible to the naked eye. A quarantine period for all newly purchased fish to check for signs of illness which includes: flicking or scratching, clamped fins, gasping, fatigue, staying at the surface, white spots, fluffy white fungus or sitting at the bottom of the aquarium as oppose to swimming around. Share a picture of the symptomatic fish on a hobbyist forum, with your LFS or an industry expert, research the feedback, then treat the whole tank with the correct disease treatment or methods for the right volume of water and duration per manufacturer's directions.

Oxygen Levels

If the fish was gasping at the surface before passing away, it may have succumbed to low oxygen levels. Most aquarium fish take in oxygen via their gills and without sufficient dissolved oxygen they can suffocate. If a lack of oxygen is suspected, check that the filter and pumps are working properly. Water surface agitation is typically all it takes to encourage gas exchange, driving off CO2, so increasing flow or adding an airstone and airpump will help promote suitable exchange.

Circulation pump and/or an air pump with air stone can help improve oxygen levels

Other causes of low oxygen levels to note: warm water holds less dissolved oxygen than coldwater, overstocked aquariums may deplete oxygen through inhabitant respiration, and biological filter bacteria may consume oxygen.


Fish have evolved to live within a certain temperature range. This range affects their metabolism, growth, ability to feed and can encourage breeding. Tropical fish need temperatures around 78°F in comparison to goldfish which can go as low as 40°F. However, no fish can go from one temperature to a radically different one quickly, as the shock can kill them; keep a tropical fish in cool water for too long, it will eventually die. So research the temperature requirements of the fish in your care or plan to keep and use a thermometer to monitor the temperature at all times.


Bullying in the tank is due to incompatibility of fish and can a new fish to succumb to being beaten to death or jumping out of the aquarium

Another common cause of death in newly introduced fish is that it may have been attacked, chased, stressed, or bitten by a established resident until it gives up and eventually dies. So before choosing a new fish to purchase, ask some questions and share tank details at the LFS. Research fish compatibility, especially with cichlids, then observe the newly introduced fish for the next few hours and days after being introduced. The new fish can be placed in an acclimation box or in-tank refugium that sits within the aquarium to allow resident fish to get used to the new fish over a period of weeks, so that the new fish can be released safely. During this time, observe the aggressiveness of the residents, and have a plan for what to do if the residents' aggressiveness doesn't decrease over time. If they don't, unfortunately, the new fish may not be a good fit for the aquarium.

READ MORE about dealing with bully fish, here: Dealing with Aggressive Tank Mates when Introducing a New Fish


fish waiting for food

Starvation is another one of top causes of death and often occurs with wild caught fish in the trade, as they may have not been quarantined and acclimated to aquarium diets like flakes or pellets. Observe new fish at mealtimes to see if it eats anything. If it doesn’t, try other foods until it does, including frozen or live foods. Also make sure food isn’t gobbled up other fish which are aggressive eaters. Check fishes belly making sure its belly or eyes aren’t sunken in. Speak to other keepers and read up on the species and their diet.   

Final Thoughts

These are some of the top causes of fish death in the aquarium, other factors should be considered such as: mishandling, overfeeding, lack of maintenance, or even introducing contaminants in the aquarium with your hands. Many can of these causes can be avoided with proper observance and knowing the signs. If your fish have experienced other causes, please feel free to share to raise further awareness and strengthen the hobby.