New Tank Syndrome

You may have heard of the term new tank syndrome (NTS) before; it is what happens when the fish tank is not cycled properly before adding the fish. It’s called new tank syndrome because its normally happens to very new aquariums, but it can actually happen to any aquarium. It will occur either shortly after new fish have been added or when you install a new filter.

If something goes wrong and your tank does get NTS; don’t be too hard on yourself, it happens to every fish keeper at some point. My first tank experienced NTS, but not during the setting up phase, it was when I added some new fish a few months later. 


If you have been testing your water every day you will notice the level of ammonia and nitrite shoot up.

If you have not been testing your water you will probably notice the following things.

  • White cloudy water
  • Your fish will look sad and tired. They may be sat on the aquarium floor or right at the surface of the water.
  • You have found dead fish in your tank.


If you have already read the fish tank cycle page you will know that to cope with the waste the fish produce the aquarium’s biological filtration relies upon there being loads of friendly bacteria within the aquarium. New tank syndrome occurs when there are not enough bacteria to cope with the waste being produced.

It normally happens when the first lot of fish are added, but it can happen at other times as well;

  • The filter was turned off for more than a few hours, bacteria within the filter will start to die if the filter is switched off. This can be a big problem for fish keepers moving house.
  • The filter was blocked; make sure you check the inlet and outlet for your filter when performing your aquarium maintenance.
  • The aquarium is overstocked
  • The fish have been overfed
  • The filter media was cleaned in tap water, the chlorine and chloramine in tap water will kill the friendly bacteria that have grown on the filter media, this will put you back to square one again.
  • You replaced too much of the filter media at once, filter media does need replacing eventually, but if you do need to replace it try doing a bit at a time. I understand smaller filters may only have one bit of foam. For these filters you could try cutting the old and new foam in half, therefore using 50% old foam and 50% new foam. Doing it this way will give the bacteria time to regroup.
  • The aquarium was cleaned to thoroughly; friendly bacteria don’t just live in the filter they live on all the aquarium surfaces including the gravel. Therefore cleaning all the gravel and doing a complete water change all at same time will actually cause the fish more harm than good.

Possible Treatments

How you should go about treating new tank syndrome depends on the levels of ammonia and nitrite in the water and how your fish are reacting.

You need to start by testing your water if you have not already done so. A reading for ammonia and nitrite of 1.0 ppm could kill your fish and will require more drastic action. The problem with drastic action is that it will hinder the aquariums natural cycling process and prolong the period of time your aquarium is suffering from NTS.

  • One option is to do a large water change. You will need to make sure the new water has the same chemistry (pH, hardness and temperature) as the current aquarium water or you could cause your fish more stress. While a large water change will remove harmful ammonia and nitrites it will also remove the good bacteria which have been growing in that water. In the short term it may save your fish, but it will mean the aquarium is going through the cycling process for longer.
  • If the fish have started dying or are looking very sick I would suggest, if possible finding them a temporary home while their aquarium is going through new tank syndrome. You may have a friend with an aquarium who could help you. If not try talking to the shop you bought the fish from. Many large aquatic shops offer a fish boarding service. I used such a service when I moved house. I put my fish into boarding at Water Zoo in Peterborough (England) for around 3 weeks, while I got the aquarium ready for them at my new house.
  • If your test results gave lower readings and your fish are coping ok, I would suggest you try the following things.
  • Stop feeding your fish for a few days. Of course do not stop feeding them forever, but fish can go for quite a while without being fed. The more you feed them, the more ammonia will be in the water.
  • Add some liquid bacteria, it won’t make everything instantly ok, but give your biological filtration a bit of a boost. I’ve always used Nutrafin Cycle, but alternatives are available.
  • Perform small water changes.

However you try to treat NTS always remember to keep testing your water, only by testing your water will you know when the water is safe.

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