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Why do my fish keep dying in my newly set up aquarium?

Updated On September 23rd, 2025

We have added fish to our newly set up aquarium twice now and both times the fish were dead or dying within 24 hours. The pH is at the proper level. The tank was filled with spring water. Why do our fish keep dying?

3 Answers

Most Helpful Answer

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Answered By Angel Alvarado, LVT

Licensed Veterinary Technician

Published on January 1st, 2019

It is possible the tank has not cycled properly which results in high ammonia and nitrite levels which can kill fish. Allow the tank to run, without fish, for at least 4-6 weeks. During this time, use a home test kit or submit a water sample for testing every few days. The local Petco may do this for free or for a small fee. What you are looking for is to have ammonia and nitrites at 0 ppm and nitrates at 20ppm or less. pH can hover around 7 for most community fish. Avoid large, rapid swings in pH though. What is happening in the tank during this time is that beneficial bacteria are populating the gravel/substrate and the filter. These bacterial populations are responsible for the Nitrogen Cycle. This cycle breaks down wastes from fish, uneaten food and plant matter into by-products that are filtered out or dissipate. You can "cycle" a tank with fish, however this increases the risk of loss. Once your tank has cycled, you can add fish. You will still need to monitor water chemistry and perform partial water changes regularly. Use only conditioned water that is temperature matched to prevent stress or shock. You will also need to vacuum the gravel regularly and rinse the filter media. Never use tap water to rinse. Use a bit of tank water to protect bacterial populations in the sponge.

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Other Answers

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    Answered By Angel Alvarado, LVT

    Licensed Veterinary Technician

    Published on May 22nd, 2017

    The quick start liquid will add nitrifying bacteria but will not eliminate ammonia or nitrites. Both are harmful and can kill fish in high enough concentrations. You can use a home test kit to test the water or you can submit a sample for testing. Most pet stores or aquarists will do this for free or for a nominal fee. There are chemical additives you can use to decrease levels. When adding water, use treated water or aged water. Ideally, the tank should be left empty for about 4 weeks to properly establish the nitrogen cycle before adding fish. At this point, you may lose the second fish regardless of treatment. Test the water, make adjustments, let the tank cycle and start fresh. Adding an airstone can increase oxygen levels.

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    Answered By Angel Alvarado, LVT

    Licensed Veterinary Technician

    Published on June 30th, 2018

    Check the water chemistry first. Use a home test kit or submit a sample of water for testing. Most pet stores will do this for free or for a small fee for. High ammonia or nitrite levels can be toxic to fish. Aim for 0 ppm ammonia and nitrites with a nitrate level of 20ppm or less. Perform a partial water change, if indicated, to stabilize chemistry and maintain water quality. Use only aged or conditioned water that is temperature matched to prevent stress or shock. Check the water temperature as well, confirming it is appropriate for your species of fish.

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