New Tank Syndrome in Fish

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What Is New Tank Syndrome in Fish?

New tank syndrome is the most common cause of death for fish in new fish tanks. It occurs in every new aquarium that does not have established biological filtration.

Biological filtration is made up of beneficial bacteria living in the tank’s system that converts toxic fish waste into safer products. It can take between four to six weeks for your filtration to becoming fully established or mature. Basic water tests will tell you instantly if your tank is at risk so you can correct it before your fish die.

Signs of New Tank Syndrome in Fish

Fish experiencing new tank syndrome may exhibit:

  • Lethargy

  • Decreased appetite

  • Increased slime coat/cloudy appearance

  • Death

  • Cloudy and foul-smelling tank water

Typically, larger fish will show clinical signs more quickly since they’re exposed to more toxins, given their larger gills. However, all species will have different tolerances for different water parameters, which can predispose some fish to become sicker faster than others.

Causes of New Tank Syndrome in Fish

New tank syndrome occurs due to a lack of biological filtration. In all aquatic systems, your biological filtration is made up of beneficial bacteria living in your tank substrate and filtration. These bacteria convert the primary fish waste (ammonia) into nitrite and, finally, nitrate. Ammonia and nitrite are highly toxic to fish, but nitrate is relatively safe.

It can take four to six weeks to build your fish tank’s biological bacteria to a level where they can handle your fish’s waste. During that time, your fish tank is “cycling.” You will notice a progression in your water tests from an ammonia spike to nitrite to nitrate.

How Veterinarians Diagnose New Tank Syndrome in Fish

New tank syndrome is easily diagnosed with a history and basic water quality testing.

  • History: The tank filter is brand new, recently replaced, or has been dry or without oxygen for too long.

  • Water quality testing: Tests for ammonia, nitrite, and nitrate will be performed (which you can do yourself at home). Results will show high ammonia, none or minimal nitrite, and no nitrate.

A sudden change in the water’s pH can also cause new tank syndrome. With a sudden pH swing, your bacterial colonies will die, causing a build up of ammonia. This can be confirmed using a pH test in addition to the ammonia, nitrite, and nitrate tests.

Treatment of New Tank Syndrome in Fish

The best and only true treatment of new tank syndrome is water changes and patience. There are many over-the-counter treatments that supposedly instantly start your nitrogen cycle, but these have been proved to be ineffective.

Ammonia-binding products can stall out your tank’s filtration because it will starve the beneficial bacteria, preventing their growth. Removing water from your tank and replacing it with new water, either treated tap water, bottled water, or balanced reverse osmosis water, are the best method to keep your tank from becoming too toxic.

It's important to check your water chemistry daily and never replace more than 50% of the total volume at one time. During the first four to six weeks of starting a fish tank, you should test your ammonia, nitrite, and nitrate daily or every other day. If any of your water chemistry parameters listed below are above range, you need to remove some of the existing tank water and replace it with new water from one of the sources stated above.

  • Ammonia >0.1 mg/L

  • Nitrite >0 mg/L

  • Nitrate >20 mg/L

Always treat your tap water with a water conditioner that treats both chlorine and chloramine, and make sure your water temperature matches your tank prior to adding fresh water. If your tap has high ammonia levels, you may need to switch to bottled water until your filtration is established and can handle incoming ammonia.

Recovery and Management of New Tank Syndrome in Fish

If your tank tests positive for new tank syndrome, an immediate 50% water change is the best chance your fish have for survival. If your tank pH differs from your tap pH by more than 0.5, you will have to do a smaller water change.

Test your water chemistry daily or every other day to ensure your ammonia and nitrite levels stay low. You will likely need to do more water changes depending on how many fish you have in your tank and how much they eat. If they are eating lots of protein, you can expect your ammonia levels to be higher because nitrogen is a main component of protein. Never fast your fish to short cut your nitrogen cycle; constantly swimming requires energy, which your fish need from their diet.

Other than improving your water chemistry, there are no additional treatments for new tank syndrome.

Preventing New Tank Syndrome

The best way to deal with new tank syndrome is to prevent it as much as possible. Whenever you start a new fish tank, you can try to steal some already established filter media from a system with similar species. Or, start with just a small percentage of the total amount of fish you plan to add. Starting a tank with fewer fish and slowly increasing their number will allow your filter to establish itself gradually and not cause an ammonia spike.

New Tank Syndrome FAQs

How long does new tank syndrome last?

Expect new tank syndrome to last four to six weeks until your biological filtration is mature and cycling completely. Warmer tanks will cycle faster than colder ones.

How do you clear up new tank syndrome?

The only way to fix new tank syndrome is with regular—possibly daily—water changes with fresh water.

Can fish recover from new tank syndrome?

If caught early and treated quickly, yes, most fish will be able to recover from new tank syndrome. Secondary issues with bacteria and parasites are common due to the stress of new tank syndrome. These may require additional treatment from your veterinarian.

What is the difference between new tank syndrome and old tank syndrome?

New tank syndrome happens with all new tanks and occurs when the biological filtration is not yet established. Old tank syndrome occurs when your carbonate alkalinity (KH) reaches close to 0 mg/L, leading to a pH decrease. This pH decrease can kill off your biological filter and lead to an ammonia spike. The low pH (6.0 or less) will protect your fish from the toxic ammonia due to its conversion to a non-toxic form.

Featured Image: niuniu/iStock via Getty Images Plus

Jessie Sanders, DVM, DABVP (Fish Practice)


Jessie Sanders, DVM, DABVP (Fish Practice)


From the love of animals and the underwater world came the most unique and amazing veterinarians on Earth, Dr. Jessie Sanders. Dr. Sanders,...

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