Why Do My Fish Die After Water Changes?

Tamoha Saha
PhD candidate, Veterinary Medicine

Water changes, though necessary for a habitable environment for your fish, can also be one of the most stressful parts of their time with you. Like any animal, fish are sensitive to drastic changes in their environments. Even if these changes are better for their health, such adjustments take time and gradual introduction to best acclimate your fish to their environmental changes.

The Reasons of Fish Dying after Water Changes

While regular water changes are in your fish’s best interest, there are specific standards to follow to ensure that your water changes are more helpful than harmful. Ask yourself the following questions to determine if your water change process can be adjusted to ensure the survival of your fish.

Is My New Water the Appropriate Temperature For My Fish?

If the new water is too cold or too warm, the overall temperature of your tank will not be appropriate for your fish, and can take extended periods of time to return to the appropriate temperature. Even if there is already appropriately heated water in your tank, new water will change the temperature of the entire tank. This can lead to stress and shock, which may also lead to fish death. Assure that your tank water is always the optimal temperature by:

  • Measuring the temperature of your new water and assuring it is no more or less than 2 degrees from the desired temperature before using it in your water changes.
  • Using an accurate thermometer to continuously monitor your water temperature, and routinely assuring that the temperature is appropriate.

Is There Too Much New Water Introduced During Water Changes?

If your fish have remained healthy before your water changes, and their water has remained within the desired parameters for your fish, it is recommended to gradually combine your new and old water over time, until most of the water is new. Introducing a large amount of new water can contribute to the stress and shock of your fish, and potentially lead to their death. Assure that your water change is comfortable for your fish by:

  • Introducing new water cup by cup, over a period of 3-5 days.
  • Leaving a large portion of old water, up to 3/4th of your tank’s volume, in your tank when performing large scale water changes.
  • Performing smaller water changes more frequently rather than large water changes less frequently, so your fish become attuned to water changes without experiencing a drastic amount of change.

Is My New Water Changing My Old Water?

Measure the parameters of your water before and after water changes – especially ammonium, nitrite, and oxygen levels, and pH values. In saltwater tanks, salinity should be taken into account as well. Do any of these parameters significantly change after water changes? Even if the parameters change to become more appropriate to your fish, your fish will still sense and respond to the changes in their environment with stress and discomfort. If you do want to adjust your water quality over time, do so after your fish have acclimated to your water change, and do so gradually.

If your new water is causing drastic changes, correct them by treating your water to align with the parameters you have measured before changing your water.

Inappropriate parameters can lead to osmotic shock, which can then lead to the swelling and bloat of affected fish.

Does My New Water Have Inappropriate Chemicals in It?

Tap water is often treated with chlorine, fluoride, or other chemicals that allow for the water to be consumed by humans. While this is appropriate for mammalian ingestion, fish may not be able to tolerate these chemicals. Your new water should be treated to eradicate these chemicals to avoid chemical imbalances when introducing the water to your tank.

Are My Fish Healthy?

If you have completely mastered the art of the water change and cannot find a single flaw in your process, yet your fish continue to respond poorly to water changes, the issue may lie in the health of your fish. If you bought your fish from a questionable source, or have recently had a bacterial infection with lingering after effects, your fish may be weakened and less tolerant to a changing environment.

Assure that your fish are healthy before introducing them to any drastic changes, including water changes or new tank-mates. This entails careful observation of their behaviors, appearances, and appetites before, during, and after water changes. If you notice a decline in their activity or a worrisome appearance, consider isolating and quarantining the fish of your concern, as well as delaying significant changes until all affected fish are healthy and able to tolerate a water change.

With all of these stipulations to water changing, one may wonder - What is the Point of Changing Water?

Because your fish are restricted to a consistent volume of water, even the most well kept water becomes less efficient at oxygenating its fish. Fresh water will lead to more effective filtration and bacterial inhabitants, so it is absolutely necessary for appropriate aquatic husbandry. The buildup of chemicals and biological waste is not diluted in vast bodies of water, and often has nowhere to go in an enclosed aquarium. Water changes are meant to mitigate the effects of living in smaller volumes of water.

If you notice your fish’s health declining soon after a water change, you may be able to prevent their death by acting quickly and effectively.

If you have any of your old tank water left over, mix a 1:1 solution with new, appropriately treated water, and isolate any and all of your ailing fish. Identify what is different about your fish – are they bloated? Eating too much or too little? Appearing malnourished? Usually, the problems you identify may also lead you to what aspect of your water change may be negatively impacting your fish. Continuously monitor the parameters of all of your tanks, and seek veterinary advice if your efforts are still not solving your problem. As resilient as fish can be, rare and uncommon illnesses may inflict even the most experienced hobbyists – medical care may be your last and most effective option.

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