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What is Old Tank Syndrome? – Symptoms, Causes and Cure

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I was under the assumption that my tank was perfectly in balance, because it had been running smoothly for a long time. When I added a new fish, it would die within days. I thought it was a weak fish, but my local fish store told me it was old tank syndrome. I went online to check and it indeed turned out to be old tank syndrome.

Old tank syndrome is an accumulation of ammonia, nitrite and nitrate together with a drop in pH because of a lack of regular maintenance. An aquarium that has been running for a long time seems to be in balance and healthy, but the water parameters are slowly turning for the worse.

If you have realized your aquarium has old tank syndrome, you are already half way there. In most cases, the owner is unaware until he or she adds a new fish.

I will tell you everything you need to know about the symptoms, the causes the cure and how to prevent it in the future.

What Is Old Tank Syndrome: Symptoms and Effects

These are the symptoms of old tank syndrome. Do you recognize them in your own aquarium?

  • pH has lowered
  • nitrates and phosphates have accumulated to a high concentration
  • possibly measurable ammonia and nitrite levels (toxic at low concentrations)
  • new fish that are added to the aquarium pass away

Chances are that you realized there was something wrong with your tank in the same way that I did. You added a new fish and it sadly did not survive. I even called my local fish store if they experienced more problems with these fish.

New fish dying is the most common symptom of old tank syndrome.

The reason why the new fish died is because the water in your aquarium is not inhabitable for new fish. The pH is acidic, it can even be 6 or lower, and there is a measurable concentration of ammonia and nitrite in your tank.

The old fish that are living in your tank have grown accustomed to the unhealthy water parameters. It probably stresses them out as well, but they are able to survive.

I have recorded and published a video on my Youtube channel that also explains old tank syndrome. You can check it out here if you want:

What Causes Old Tank Syndrome?

  • lack of maintenance (water changes)
  • topping off the evaporated water
  • slowly decreasing pH

Old tank syndrome is an assassin which you do not see coming if you are not aware. It all happens because of a slow accumulation of fish waste, uneaten food and rotting organic matter like plants.

If your tank is running well, all these ammonia sources will be dealt with. The aquarium’s beneficial bacteria colony can handle the ammonia and breaks it down into nitrite and finally into nitrate.

The breaking down of ammonia is done by beneficial bacteria in your tank. This is part of the nitrogen cycle that keeps our aquariums safe for fish.

If we, as owners, do not remove the nitrate it will build up to a point where it can become toxic to the fish. Even if we have learned that nitrate and phosphate are not toxic to fish, there comes a point where it will stress out and potentially kill the fish living in the aquarium.

With this accumulation, the pH will slowly drop. When the pH dives below 6.0, the bacteria in the biofiltration are not able to break down ammonia properly. At this point the ammonia and nitrite concentrations will rise, stressing your fish.

Because it happens gradually, the fish have time to adapt and fight the poor water quality. The weak fish will die the soonest, but most of the time the owners think this can be of old age. The stronger fish will continue to live on.

How to Fix Old Tank Syndrome

  • gradually work your way towards healthy water parameters
  • do a 10% – 15% water change every day
  • keep a close eye on the water parameters: measure them

If you have established that your aquarium has old tank syndrome, it is time to get those water parameters back to what they should be.

It is key to do this gradually, because otherwise all the fish that have grown accustomed to the terrible water quality will suddenly die from the change.

When you start doing the small daily water changes, make sure to monitor the water parameters closely! What you should see is the nitrate levels slowly decreasing because you are diluting the water. The ammonia and nitrite should also decrease, and at some point the beneficial bacteria will help you again.

If you test your water parameters and you see the ammonia shooting up, put the water changes on hold and let it settle for a day or two. This all happens because the ecosystem in your tank is so unstable.

When you are done doing the water changes, and the water parameters are back to a steady and healthy level, you can start cleaning the filter and the tank itself.

Make sure to remove all the uneaten food, poop and other sources of ammonia.

Prevent Old Tank Syndrome in the Future

  • Do your regular water changes
  • Add some live plants to help remove nitrates/phosphates from the water
  • Measure your water parameters once in a while to keep track on your parameters

Keep in mind that an aquarium is a closed system: anything that we put it stays in the tank until it is removed in some way. This is especially important while feeding.

With old tank syndrome (and all other diseases / health related troubles), preventing is way more important and effective than curing. Luckily it is also not that hard to prevent old tank syndrome.

Do regular water changes

The most important thing is to do your regular water changes. Now I know that there are a lot of different opinions on how much water you should change and how often. If you have a well established tank, I recommend doing a 20% – 25% water change once every two week.

Add live plants to your tank

If you have a lot of live plants in your aquarium that help you get rid of nitrates and phosphates, you can do less water changes. However do not forget to clean your filter once per month because there might be a lot of plant matter and fish poop that is out of sight, but not out of tank.

Live plants can be a great help in keeping your water clean, but to each their own. Just know that they do not have to be a pain.

Measure your parameters

Measuring your water values gives great insight in the condition of your tank. Just imagine if you had measured your water parameters earlier, it would have meant dealing with the old tank syndrome earlier.

If you want to measure your aquarium water accurately, and do not have a test kit yet, I recommend going with the API Master test kit. You can check out the price on Amazon here. There is also the option to measure using test strips. I always use those as a quick check-up. They are not as accurate as any liquid based aquarium test kit.

Related Questions

How do I know my tank has old tank syndrome without measuring the water parameters? This is one of the most difficult things about old tank syndrome. It is super hard to know if your tank has it, because in theory it is not visible to the eye. If you notice a new fish that you recently added has died, it is a good indicator. If you do a big water change for the first time in a long time, and your fish die, it is a good indicator your tank is (was) suffering.

What is new tank syndrome? New tank syndrome is the lack of beneficial bacteria to handle the ammonia that fish produce in an aquarium. Causes for this is when the tank is not properly cycled. Another cause can be an attempt to treat the old tank syndrome in an aquarium by doing a big water change and immediately adding new fish. There is no bacterial colony to support the ammonia the fish produce.

Pookode fish image owner: KannanVM licensed: CC4.0 no changes made

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