Why Do My Fish Keep Dying?

Tamoha Saha
PhD candidate, Veterinary Medicine

Even the most experienced hobbyists come across obstacles that result in numerous fish deaths – many fish are fickle and sensitive creatures! Most of the time, this can be corrected with a change in husbandry – the daily environment and nutrition of your fish. The process of determining the necessary change requires monitoring and observation of your fish and their environment; be sure to pay close attention to your fish’s health and behavior as you investigate!

The Main Causes of Consistent Fish Death Include:

  • Inappropriate water chemistry – imbalances in nitrite and ammonium
  • Shock due to rapid water change – drastic water chemistry changes
  • Inappropriate water quality – decreased filtration, inappropriate pH, salinity, temperature
  • Overfeeding
  • Contagious disease
  • Foreign chemicals
Overfeeding Fish
Overfeeding Fish

Remember that different fish require different environments – parameters that are appropriate for one species of fish may be uncomfortable to another. If you have multiple species of fish sharing one tank, assure that the parameters of your tank are suitable to all fish, or that each species of fish has a tank with the appropriate qualities.

Curative changes may be dependent on the type of fish you have, but there are a few basic questions you can ask to troubleshoot dying fish.

1. Have I Introduced Any New Fish to the Environment?

  • If so, have they been properly isolated and quarantined before introducing them into your main tank?

-If not, consider isolating your new fish and assessing the parameters of your tank before and after removal – ammonia, nitrogen, pH, and filtration. Has anything changed? If so, you may have found the culprit in your dying fish.

-If one fish has been identified with a contagious disease, decontaminate your tank, isolate the sick fish, and treat all remaining fish with preventative doses of medications such as antibiotics.

  • Is your tank big enough? Your tank may have reached its maximum capacity, and cannot tolerate the current amount of fish. Observe the behavior of your fish after multiple deaths without replacing the fish, and note any differences in behavior, activity, and overall health.

2. Have I Introduced Any New Chemicals/Ornaments into the Environment?

  • Some fish are sensitive to a change in brands of products, as their chemical formulas may differ even if they are for the same purpose. How do your fish react if this new chemical is replaced or removed?
  • More extreme cases of this are identified as “New Tank Syndrome,” where dramatic changes in the environment lead to imbalance and an inappropriate living environment. This can also occur when water changes are too frequent.
  • New products may need to be gradually introduced to the water, as a sudden change in water quality can lead to the shock and death of fish.

3. Has My Aquarium’s Parameters Changed Over Time?

  • Consider diligently tracking and comparing the parameters of your water quality over time, noting any decreases/increases in important values. You may find that your tank is becoming slightly acidic and needs pH correction, or your water heater has recently failed in its ability to maintain an appropriate temperature.
  • This can occur as a result of a change in environment or faulty equipment. Test your equipment in water (store bought distilled water, aquatic store tanks) with known parameters to assure they are reading values accurately.

4. Have My Fish Been Eating All of Their Food?

  • If not, remaining food may cause bacterial growth and contamination, leading to bacterial infections and therefore death in your fish.

5. I Can’t Find Anything Wrong with My Animal Husbandry – What Do I Do Now?

In the rare occasion where your husbandry practices have no identifiable areas of improvement, there are rarer, more specific circumstances that may be distressing your fish.

  • You may be using a lotion or soap that has dissolved into the water and is causing chemical imbalance. Try washing your hands with unscented soap and water or wearing gloves before performing aquatic husbandry.
  • A part of their greater environment – your home – may be affecting their health. Assess if your house’s air quality is free of toxins, there are dramatic and inconsistent changes in lighting, or loud and distressing noises.

6. What If My Living Fish Appear Unwell?

There is a chance that a contagious disease has affected your tank, and your fish must be troubleshooted and treated alongside your tank. This can be achieved with isolation, deep cleanings of your tank, and the appropriate antibiotic treatments. Do not add any more fish to your tank until your existing fish are free of any potential illness.

How Can I Prevent More Fish Deaths?

  • Regularly test the water quality of your fish. As a former aquatic laboratory technician, I would do this as often as twice a day and would quickly correct any deviations from desired water quality parameters. This allowed for minimal fish discomfort and fatalities.
  • Change water regularly, but always leave a fraction of the old tank water behind to prevent shock.
  • Assure your aquatic equipment, including measuring devices, are functioning properly. If your thermometer is reading even a few degrees too high or low, your fish are at risk of temperature related illnesses. This can be done by routinely washing, inspecting, and testing your equipment in distilled water.
  • Quarantine all new fish before adding them to your main aquarium, even if they are completely healthy. You may even introduce the water of both tanks to the other’s to allow all fish to gradually adjust to their new tank mates.
  • Gradually introduce any new chemicals/products to all fish tanks. Even if these chemicals are to correct undesired water quality parameters, any dramatic shift in environment can be dangerous to your fish.
  • Monitor the behavior of your fish, especially during feeding time. Note changes in their appetite and if leftover food has settled to the bottom of the tank. Early recognition of potential illness can save your fish’s life with appropriate action!

To reiterate, consistent fish death is most likely an issue in their current environment and care. Monitor your water quality, fish’s health, and aquatic instruments to best identify the issue. Identify any and all differences that arise when your fish die to pinpoint a potential problem, and therefore solution.

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