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Is my 5-gallon tank too small for my goldfish? Why are they dying?

Updated on September 23rd, 2025

Pet's info: Fish | Unknown - Fish

Please help!! I have a 5 gallon fish tank starter kit. Water conditioner. Thermometer. And it has a filter. I have bought 3 gold fish all from Walmart. (I know bad idea) and all have died 1 first day. 2nd one the next day. And the third is dying currently :( they swim by the filter and float then have a hard time breathing it looks like and then die. My autistic 5 yr old is devistaed and I don't know what I could be doing wrong. I did research and research and more research!! Help:(

3 Answers

Most Helpful Answer

Angel Alvarado, LVT

Answered By Angel Alvarado, LVT

Licensed Veterinary Technician

Published on September 18th, 2018

A 5 gallon tank is insufficient for goldfish. They produce a larger than normal bio-load (wastes) that can quickly overwhelm a small tank. Ideally, one goldfish should be in a 20 gallon tank with an extra 10 gallons added per additional goldfish. The filter should be rated for at least twice the volume of the tank.
They are likely succumbing to ammonia and/or nitrite toxicity. If you test the water, it is likely both parameters will be dangerously high. Perform a 50% water change. Use only conditioned water that is temperature matched to prevent further stress or shock. Goldfish are cold water fish and do best between 65 F and 72 F. Plain aquarium salt, NOT table or Epsom salt, can be used to reduce stress, improve gill function and aid in recovery. Dissolve 1 teaspoon per gallon in a cup or two of tank water and gently pour it into the tank. Treat for no more than 14 days then perform a partial water change to decrease salinity.
A 5 gallon tank would be appropriate for one male Betta or a small school of Neons (3-5).
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Related Answers from Veterinarians

  • Answered By Angel Alvarado, LVT

    Licensed Veterinary Technician

    Published on March 3rd, 2019

    Check the water chemistry and quality. This can be done by using a home test kit ( https://www.petco.com/shop/en/petcostore/product/api-freshwater-master-test-kit ) or by submitting 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. Nitrates can be 20 ppm or less. Perform a partial water change if necessary. use conditioned water that is temperature matched to prevent additional stress or shock. Goldfish are cold water fish and do best between 65 F and 72 F.
    You can add plain aquarium salt (NOT table or Epsom salt) as a general tonic. Dissolve 1 teaspoon per gallon of water in a cup or two of tank water and gently pour it into the tank. It can improve gill function, reduce stress, treat minor infection and aid in any recovery. Treat for 14 days then reassess the condition. Perform more partial water changes afterwards to reduce salinity.
    In case he is constipated, you can offer bits of cooked and shelled green peas until defecation and swimming are normal. You can soak food in garlic juice to stimulate the appetite and provide an immune system boost.

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    1Pet Parents found this answer helpful

  • Answered By Angel Alvarado, LVT

    Licensed Veterinary Technician

    Published on January 23rd, 2018

    Goldfish need powerful filtration. Your filter should be rated for at least twice the tank volume. Once you get a test kit, check the chemistry. Aim for 0 ppm ammonia and nitrites with nitrates at about 40ppm. Use a kit that has liquid reagents, not strips. The liquid kit is more accurate. The pH should range between 7.2-7.6. Being cold water fish, goldfish thrive between 65F and 72F.
    You can dissolve 1 teaspoon of plain aquarium salt (NOT table salt) per gallon of water to use as a general tonic. It will reduce stress, improve gill function, treat minor infection and aid in recovery. Treat for 10-14 days then reassess the condition of your fish. When performing partial water changes, treat only the volume of water being replaced not the entire tank volume. Change no more than 30% of the volume at a time. Use only aged or conditioned water that is temperature matched to prevent stress or shock.

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    1Pet Parents found this answer helpful

Smart Picks for Smart Questions

Vet Q & A
Why is my fish resting at the bottom and can't open its mouth?
Check the water chemistry and quality first. Elevated ammonia or nitrite levels can be toxic. Partial water changes can be done to stabilize and improve water chemistry. Use only aged or conditioned water that is temperature matched to prevent further stress or shock. For goldfish, a range of 65F to 72F is recommended. Check the filter and rinse the sponge with tank water, not tap water, to spare beneficial bacterial populations and to maintain water quality. The tank may not be large enough to support 5 goldfish. Generally, 20 gallons should be allowed for the first goldfish then a 10 additional gallons should be provided for each additional goldfish. This is due to the larger than average bio load (wastes) they produce. Consider a larger tank or decrease the number of fish in one tank. Use a filter that is rated for at least twice the volume of the tank and add an airstone and pump for increased oxygenation. Plain aquarium salt (NOT table salt). Can be used as a general tonic. It will reduce stress, improve gill function, aid in any recovery and may treat or prevent minor infection. Dissolve 1 teaspoon of salt per gallon of water in a cup of tank water and gently pour it into the tank. Treat for 10-14 days. When performing water changes, add salt only to the amount of water being replaced, not the entire tank volume.
Updated on September 23rd, 2025
Training & Behavior
Why is my goldfish suddenly aggressive and pecking other fish?
This could be due to having a tank that is too small. 6 goldfish in one tank poses several potential issues. Ideally, 1 fish should have a 20 gallon tank with an additional 10 gallons provided per extra fish. This would mean they should be in at least a 70-75 gallon tank. The reason being goldfish produce a larger than average bio-load (wastes) that can quickly overwhelm a small tank and filter. Poor water quality can lead to stress and infection as well as behavioral issues. Depending on their sizes, the present tank could be overstocked leading to bullying and fin nipping. Check the water quality and chemistry. Use a home test kit or submit a water sample for testing. Most pet stores will do this for free or for a small fee. Aim for an absence (0 ppm) of ammonia and nitrites with a nitrate level of 20 ppm or less. Perform partial water changes, if indicated, to stabilize chemistry and maintain or improve water quality. Use only aged or conditioned water that is temperature matched to prevent further stress or shock. Goldfish are cold water fish and do best between 65 F and 72 F (18.3 - 22.2 C). Make sure the filter is rated for at least twice the volume of the tank. If possible, separate out the aggressive goldfish to a different tank to prevent further bullying and injury.
Updated on September 23rd, 2025
Health & Wellness
Goldfish spazzing in 5-gallon tank? Is it the water quality?
Thanks for reaching out about Timothy and Lola. First suggestion is to get a large tank. The general rule of thumb is 1" of fish per 3 gallons of water. Test and correct water quality for pH, ammonia, nitrites and nitrates. The better the filtration, the higher the bio-load. So better filtration or a second filter could help. Good luck.
Updated on September 23rd, 2025
Health & Wellness
Why is my goldfish frantic in 5-gal tank, spitting food, high ammonia?
Thanks for reaching out about Goldie. There are two concerns here: (1) not apparently eating and (2) high ammonia. For the appetite, try to note if there is feces. If feces, then eating. If appear getting thinner, then not eating enough. For the second problem, this is probably new tank syndrome, and not allowing the biological filter to establish. Water changes are needed to bring down the ammonia as that is toxic to the fish and damaging to the gills. Probably the reason for the frantic swimming. Good luck.
Updated on September 23rd, 2025
Vet Q & A
Why is my fish struggling to swim, jumping, or lying at the bottom?
Check the water quality and temperature. Warm water will not hold as much oxygen as cool water and goldfish are cold water fish. Ideal water temp is about 21 C. Use a home water test kit to check parameters. Ammonia and nitrites should read 0ppm (parts per million) and nitrates can be 40ppm or less. If you don't have a test kit, most pet stores or aquarists will test a sample for free or for a nominal fee. Ensure adequate filtration and oxygenation. Use a filter that will comfortably handle twice the volume of your tank. An airstone will increase oxygenation. A tank hood or cover is recommended to prevent fish from jumping out.
Updated on September 23rd, 2025
Health & Wellness
Why is my goldfish not moving and staying at the bottom of the tank?
Since the others in the tank are behaving normally, this one may be sick or injured. Ideally, it should be separated into a quarantine or hospital tank Check water chemistry first, even if you've done it already. You can use 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 (0ppm) of ammonia and nitrites, with nitrates at 20ppm or less. Perform partial water changes to correct or stabilize water chemistry. Use only conditioned water that is temperature matched to prevent additional stress or shock. Goldfish are cold water fish and do best between 68F and 74F. Change no more than 30% of the volume at one time. In the absence of other symptoms or injury, medications cannot be recommended. You can use plain aquarium salt (NOT table or Kosher salt) as a general tonic to decrease stress, increase gill function and aid in recovery. Dissolve 1 tablespoon of salt per 2 gallons of water in a cup of tank water and gently pour it into the tank. Treat for 10-14 days then perform partial water changes to decrease salinity.
Updated on September 23rd, 2025
Food & Nutrition
Can my goldfish live long in a bowl with no filter and daily cleaning?
A fish bowl is acceptable but not recommended. Goldfish produce much more waste than other fish. This requires powerful constant filtration and water circulation/oxygenation to maintain adequate water quality. Furthermore, a bowl will quickly become too small for a healthy goldfish. Ideally, one goldfish requires a tank of at least 20 gallons. If you decide on a bowl, you will need to perform partial or complete water changes daily or at least every 2 days. Use aged/conditioned water that is temperature matched to tank water. There are small filters and air pumps that may fit in a good sized bowl to extend the time between water changes. Feed once a day and use a pelleted diet that sinks. Lifespan will depend on tank size, water quality and diet.
Updated on September 23rd, 2025

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