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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?

5 Answers

Most Helpful Answer

Angel Alvarado, LVT

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.
Vote Icon1Pet Parents found this answer helpful

Related Answers from Veterinarians

  • 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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    1Pet Parents found this answer helpful

  • 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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    2Pet Parents found this answer helpful

  • Answered By Angel Alvarado, LVT

    Licensed Veterinary Technician

    Published on May 28th, 2016

    Cleaning the filter and changing the water may have upset the nitrogen cycle in your tank. This is due to removal of beneficial bacteria which convert ammonia (from waste, food, urine, etc.) to nitrites. Water and filter changes can be stressful as well. If you have the means, test your tank water for ammonia, nitrite, nitrate and pH levels. Adjust accordingly using conditioned water or additives that will bind harmful compounds. You can also submit a water sample for testing. Moat pet stores or professional aquarists will do this for free or for a nominal fee.

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    25Pet Parents found this answer helpful

  • Answered By Dr. Massimo Orioles, DVM, Cert AVP, MRCVS

    Veterinarian

    Published on March 9th, 2018

    If persistent this is a very concerning but generic sign, which requires a logic but quite radical approach.
    Begin by submitting a water sample for quality testing. Most pet stores will do this for free or a nominal fee.
    High ammonia levels, PH imbalances may be responsible for that. Oxygen concentration and quality of the filters should be determined as well.
    If water quality is good and there are no major imbalances, your fish should get examined to look for any sign of bacterial/fungal infection and eventually water should be treated.

    Hope this answer was helpful, but please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.

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    2Pet Parents found this answer helpful

Smart Picks for Smart Questions

Vet Q & A
My new fish died quickly after purchase. What are common reasons?
The most common cause of death of a newly acquired fish is usually shock or incorrect water quality. Fish tanks should be set up and left running without fish for at least 3-4 weeks to establish a proper nitrogen cycle. There are products that can shorten this time period. If this is not done, there may be high levels of ammonia, chloramine, nitrates and nitrites which can kill a fish. When introducing a fish, the bag should be floated in the tank for about 30 minutes to acclimate the fish to water temperature. This can prevent shock if the water is too hot or too cold. Test your water quality with a home test kit or submit a sample for testing. Most pet stores or aquarists will do this for free or for a nominal fee. Correct any imbalances as indicated. Slowly acclimate any new fish as written above. You can request a consultation to discuss your concerns in greater detail if necessary. Buying fish and starting a tank can be quite confusing initially.
Updated on September 23rd, 2025
Vet Q & A
Why are my fish dying with bleeding, white spots, and polluted water?
I'm sorry you are having difficulties with your aquarium. There are several issues that should be dealt with right now. They are water chemistry, aquarium size and choice/number of fish. Any new aquarium should be set up and, ideally, run empty for several weeks (usually 4-6 weeks). This is so beneficial bacteria have an opportunity to populate the substrate and filter. These bacteria are necessary to break down wastes that fish will produce. The technical phrase for this is "establishing the Nitrogen cycle". These bacteria will break down wastes into ammonia then nitrites then nitrates. Nitrates can then dissipate or be absorbed/used by live plants. In high enough concentrations, ammonia and nitrites are toxic and can kill fish. Cycling a tank with fish is possible but it increases the risk of loss. Water chemistry should be checked every few days using a liquid home test kit. A water sample can be submitted for testing , but is not as convenient as testing yourself. Most pet stores or fish stores will do this for free or for a small fee. Partial water changes, no more than 30%, can be done as needed to stabilize chemistry and maintain water quality. Choices for water conditioners include NovAqua, AmQuel Plus, Prime, StressZyme or StressCoat. Tank size is another concern. With most fish, the widely accepted "one inch of fish per gallon" will be sufficient. For koi and other carp species, including goldfish, the rule is not sufficient. At full size, goldfish need 20 gallons per fish with an additional 10 gallons for each additional fish. Koi do best in larger tanks, much larger...we're talking in the hundreds of gallons or small ponds. These species produce a larger than average bio-load (wastes) that can quickly affect water quality in a tank. To combat this, large water volumes and powerful filtration are required. Any filter should be rated for at least twice the tank volume. I'm not sure what size tank you have now, it may be fine if the fish are young and small, but they will quickly outgrow it. The last issue is choice of fish. Koi and goldfish are cold water fish. They do best in water between 65F and 72F. Most species of sharks do best in the high 70sF to low 80sF. Keeping fish in incorrect water temperatures will lead to stress and illness. This could be compounded by the water quality/chemistry issues you are having currently. Until the chemistry is stable and water quality is maintained, your fish will continue to become ill and possibly die. Any treatment will not be successful in this situation. Do not purchase or add any more fish until the tank has cycled. If you have additional questions, you can post them in the forum or request a consultation to discuss the case in greater depth.
Updated on September 23rd, 2025
Health & Wellness
Why are my fish dying with pink and green algae in their tank?
Check the water chemistry using a home test kit or submit a water sample for testing. The local Petco may do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with nitrates at 20ppm or less. Perform partial water changes to stabilize chemistry and/or to improve water quality. Rinse the filter media in tank water, not tap water, and replace it. Use only conditioned water that is temperature matched to prevent stress or shock. To limit algae growth, maintain pristine water quality via filtration and regular water changes. If possible, turn up filter flow to keep water moving. If this stresses the fish though, do not do it. Do not overfeed and remove any uneaten food promptly. Limit lighting to 8 hours daily and closely monitor water temperature as well. All these can promote algae overgrowth.
Updated on September 23rd, 2025
Vet Q & A
My fish died. How do I make my aquarium water safe for new fish?
I would recommend to have the water completely changed. New water should be ideally submitted for quality testing. Most pet stores will do this for free or a nominal fee. High ammonia levels, PH imbalances may be responsible for that as well. Oxygen concentration and quality of the filters should be determined as well. Please do not hesitate to contact us again on the forum or by requesting a consultation if you have any more questions or to discuss it any further.
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Vet Q & A
My new fish died after tank cycling. My betta has fin rot. What to do?
I recommend to start with having the water tested. Many times this is offered by the pet store. If all the fish have passed away, the culprit is most often poor water quality, a disease or the tank not being properly maintained for the species of fish. I do not recommend to get more fish until you know the water conditions are correct and that you know the environment (temperature...etc) is appropriate for the species of fish you want to have. I hope this helps.
Updated on September 23rd, 2025
Vet Q & A
My Tetra fish keep dying. What's causing this despite my best care?
I am so sorry that you are having this problem! With multiple die offs of fish, one of the first things to consider is that the water quality may need to be addressed. I didn’t see you mention in your post whether you have been able to assess that or not, but it would definitely be something to evaluate. Things like high ammonia, high nitrites, and high nitrates can be devestating for fish. Also, if your filters are not working properly, or are not in the correct set up, that can also contribute to fish death. From the clarity of your water and the lack of substrate and tank furniture, it makes me more suspicious of this contributing to your issues. The other major reason for multiple fish deaths are infectious diseases (bacteria, parasites, fungus) - but ideally these would be diagnosed by a veterinarian. There are many over-the-counter medication treatments at pet stores for these, however they can be damaging to your water system, so ideally would not be used unless you know the problem that is going on.
Updated on September 23rd, 2025
Vet Q & A
Is bottled bacteria the best way to quickly cycle a goldfish tank?
Hi and thanks for your request. To better control Nitrite and ammonia spikes it is best to cycle it for a month at least and measure nitrites, ammonia and nitrate constantly. Please follow the recommendations here below: The nitrogen cycle is the process of various bacteria converting harmful waste. It involves 4 steps: The first step is the decay of waste products of fish, plants, and invertebrates, along with any dead organisms or uneaten food. As these materials decay, ammonia is produced, which at even low levels will burn the gills of fish and choke off their oxygen supply. Bacteria called Nitrosomonas consume this ammonia and, in the process creating another chemical byproduct called nitrite. Although nitrite is toxic (preventing blood from carrying oxygen), fish can withstand roughly twice the amount of nitrite in their water when compared to ammonia. Next, other bacteria called Nitrobacter consume the nitrite and, in turn, release a less toxic chemical called nitrate. Nitrate requires anaerobic conditions to turn it into harmless nitrogen gas. The parameters needed to create this condition are not commonly present in most aquariums. Hence, water changes are necessary to dilute nitrate. The Nitrogen Cycle in New Aquariums Because new aquariums lack the colonies of bacteria that are necessary to provide biological filtration, the aquarium must be cycled. Cycling refers to the process of establishing and maturing the biological filter. Here's how it works: As the first fish in the new system begin to thrive, they produce ammonia. Without any Nitrosomonas colonies established to consume this toxin, levels climb and spike until the ammonia-loving bacteria population catches up. This phenomena can be observed by the naked eye as a sudden and temporary appearance of wispy, smoke-like cloudy aquarium water Ammonia levels then start to decline once the rate of ammonia production is less than the rate at which it is broken down by the bacteria. Nitrite goes through a very similar spike. Nitrite is produced through the biological activities of Nitrosomonas as they consume ammonia. As their numbers increase, so does the amount of nitrite, and in turn the population of nitrite-hungry Nitrobacter will begin booming from the abundance of nutrients. Nitrite levels will rise until the number of bacteria has increased to the point at which they break down the nitrite faster than it is being produced. The end product of this whole process is nitrate. Nitrate, in low concentrations, is not as toxic to fish and invertebrates, but can cause other problems in the aquarium. The best way to control nitrate is through regular partial water changes. Typically, new aquariums can be cycled in 2 to 6 weeks, but the actual length of time depends on many factors. Use a quality test kit to test your water conditions every other day, and when both ammonia and nitrite levels return to zero, you'll know the cycle is established.
Updated on September 23rd, 2025

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