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What is old tank syndrome? (and how to fix it)

Old tank syndrome is a problem that can easily sneak up on both new and experienced fish keepers.

This guide at Aquarium Blueprints will help you prevent this issue as well as provide tips on how to fix it.

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What is old tank syndrome?

Old tank syndrome occurs when the water in your fish tank has aged to a point that it become vastly different when compared to the original parameters. This usually means that your nitrate levels are unusually high and/or your pH is unusually low.

Old tank syndrome does not happen overnight. Instead, the higher nitrates and more acidic water slowly buildup over time if you aren’t doing enough thorough maintenance in your aquarium. Because of this, most of the fish in your aquarium will actually acclimate to the less-than-ideal water parameters.

With that said, your fish could suffer long term health problems when kept at high nitrate levels and/or pH that are more acidic than their preferences.

How to identify old tank syndrome

The easiest way to identify old tank syndrome is with a water test. For nitrate, you should keep the ppm to below 40 (if you are keeping more sensitive fish, then you might want to aim for under 20 ppm). If you noticed that the ppm is way above 40 even though you have been doing regular water changes, then that could be a sign that your aquarium has old tank syndrome.

You should also test the pH level. Before testing your tank water, you should test the source you used to add new water to your tank. The ph level should be close although your tank water should be slightly more acidic due to the organic matter being produced by the inhabitants. If you noticed a huge difference (and you aren’t intentionally changing the pH), then that could be another sign of old tank syndrome.

If you don’t have a testing kit, we recommend the API Master Test Kit, which you can buy on Amazon with this link. (#CommissionsEarned)

Without a test kit, you will have a hard time diagnosing old tank syndrome. As we mentioned above, because this phenomenon occurs slowly over an extended period of time, your fish will acclimate to the parameters. If you decided to add new fish to your community, however, they will have a harder time with the unsafe water parameters.

How to prevent old tank syndrome

The best cure for old tank syndrome is preventing it from happening in the first place. As we mentioned in our water change guide, we recommend testing your water in your tank before doing any water changes. Assuming that your tank is properly cycled (which means you have 0 ppm of ammonia and nitrites), you have to make sure that the nitrate levels are safe for your fish. The amount of nitrates you have in your aquarium should determine the percentage of water you need to switch out.

If you noticed that your nitrates have been increasing even though you are changing out a significant amount of water, then the next step is to clean your filter. To clean, make sure to use the water from the aquarium you plan on throwing out because tap water contain chlorine and chloramine that will kill the beneficial bacteria that have been growing on your filter media. Rinse out as much as the detritus from the filter media as possible through the old, outgoing tank water.

We also recommend adding live plants as they will consume the nitrates of your water column. Keep in mind that you should remove any dead or dying leaves as they will leach ammonia (which will eventually convert to nitrites and nitrates) to your tank. Some easy live plants that are resilient include Java Fern and Java Moss.

Your substrate may also contain a lot of detritus. So try to scoop as much of it up using a net or airline tubing via a siphon. If you want to siphon, we recommend getting the Python No Spill Clean and Fill Aquarium Maintenance System, which is available on Amazon. (#CommissionsEarned)

Nitrate spikes can also occur if you overfeed your fish. If you see food floating on top of the water and/or lying on the substrate after around 5 minutes of feeding (and your fish aren’t interested in eating more), then you might want to consider feeding less during the next meal time.

Last but not least, you should remove dead fish immediately from your fish tank as it can quickly foul up your water column by causing ammonia, nitrite and nitrate spikes.

To prevent old tank syndrome, your goal should be to keep nitrate levels to below 40 ppm after ever water change. This mean that you may have to change more water and/or clean more of your tank in some instances.

How to fix old tank syndrome

If you believed that your aquarium is already suffering from old tank syndrome and want to cure it, we recommend doing a water change of around 10% to 25%. We don’t recommend bigger water changes as the drastic swings in parameters can be harmful to the fish in your aquarium who are used to the old tank syndrome parameters. You should also clean out any detritus from your filter media and/or substrate during your water change (as we mentioned above, you should use old, outgoing water to rinse off the buildup) if you find any substantial buildup.

We don’t recommend cleaning everything at once as that could result in significant shifts in nitrate levels. Instead, you should focus on rinsing wastes off of the filter media during one water change and then rinsing the decorations/ornaments in the next maintenance.

When doing water changes, you should use a water conditioner (we recommend using Seachem Prime, which you can find on Amazon (#CommissionsEarned), as it will also detoxify nitrates for up to 48 hours per dosage) before adding new water to the tank. Make sure to dose base on your entire tank volume (i.e. you should add enough Prime for a 40 gallon tank even if you are only changing out 25% of the water).

After performance maintenance, test the water parameters of your fish tank. Keep a log of the readings, especially the pH and nitrate levels. You should do water changes and water tests daily. Your nitrate level should go down and pH levels should go up steadily day by day.


Old tank syndrome occurs when you fail to thoroughly keep your tank water pristine, which leads to a buildup of nitrates and more acidic water in your tank.

To prevent this phenomenon, you should test your water parameters regularly to show how much water that you should switch out during a water change. You should also occasionally clean out any detritus from your filter media and substrate.

If your aquarium is experiencing old tank syndrome, you should do daily water changes of 10% to 25%. At the same time, remove as much as the detritus from your filter media and substrate as possible.

Keep doing the water changes and test your water parameters afterwards until you get the nitrate and pH to safe levels for your fish.