Last Updated on February 10, 2023 by Matt

Almost the opposite of new tank syndrome, old tank syndrome is so called as it primarily affects older more mature tanks.  

It can be a silent but steady syndrome which will eventually result in the death of your fish. The trouble with old tank syndrome is that there aren’t many outward signs of it affecting your aquarium.

In this article we will run through what exactly old tank syndrome is, the signs and symptoms, causes, treatments, and preventions.


What is Old Tank Syndrome

What is Old Tank Syndrome

Old tank syndrome is the name given to the changes in water chemistry that often occur in older more mature tanks. 

In this phenomenon, the water chemistry will have changed significantly since you first started the aquarium. Nitrate and phosphate levels will be significantly higher than they should be. The pH will have changed considerably as well, most often becoming more acidic over time. The water hardness, both carbonate hardness (KH) and general hardness (GH), will also be out of kilter and at a different level than they should be. 

If you test the water, and see acidic water, high nitrate, high phosphate, and very low KH, and very different GH than what you should have, then you may be suffering from old tank syndrome.

Symptoms of Old Tank Syndrome

What is difficult in old tank syndrome is that there might not be many outward signs of trouble. 

As the changes in water chemistry happen over time, the hardy fish survive while the weaker fish will die off one by one as they succumb to the poor conditions. At the time you might just think a fish dying us a one off, not knowing it is down to the conditions. Eventually even the hardiest of fish will be overcome by old tank syndrome. They will be much more susceptible to disease and their life span will be shorter. 

A clear sign that something isn’t right with the chemistry of your tank is if you introduce new fish into the tank and they either become extremely unwell or die. The shock of going from good water conditions in the shop tank to poor conditions causes their body to go into shutdown.

Another sign of old tank syndrome and that you should test your water is excess algal growth or an algal bloom. As nitrate and phosphate levels are increased in old tank syndrome, algae can become quite prolific. Both nitrate and phosphate are important plant fertilizers and a source of nitrogen and phosphorus respectively. Algae will happily use these compounds for their own growth, and you may see a significant amount of growth. 

Causes of Old Tank Syndrome

Causes of Old Tank Syndrome

They’re are a few reasons why your aquarium may develop old tank syndrome.

The name holds a hint for the main reason; it’s because the tank is old and so waste and contaminants have built up. 

If waste products have been able to gradually build up to dangerous levels it is most likely because you aren’t doing enough maintenance on the tank. Aquariums need to be cleaned every week, but cleaned properly too. You need to remove the debris from the bottom of the tank using an aquarium vacuum cleaner, and you should be cleaning the decorations and glass as well. Every two weeks you should do a 25% water change, or you can do a smaller water change each week. The filter needs to be cleaned and maintained properly too.

It can be very easy to let cleaning slide a little, but it is vital to have a schedule and stick to it. Remember everything that goes into the tank stays in the tank unless you remove and clean it! 

Carbonates are normally present in some concentration in aquariums, and are critical in buffering the water to pH changes. Carbonates are also used as an important source of carbon for plants and bacteria. As carbonates are used up in old tank syndrome, the water will become more acidic over time, but will also be prone to swings in pH level. Most fish aren’t able to handle large swings in pH without serious health issues, both acute and chronic. Carbonates are also used by fish in osmoregulation, and very low levels can cause this system to be out of kilter, causing severe issues. 


To treat old tank syndrome you need to treat and work to rectify the imbalances in the water chemistry. 

If the water has become more acidic and the carbonate buffer that is measured in the KH measurement is no longer in the tank then you need to replace it. There are various carbonate buffering products that you can purchase for both freshwater and marine aquariums which will replace the carbonate ions and increase pH. 

As with most water chemistry issues, water changes should be your first port of call. However unlike ammonia poisoning, you shouldn’t do a large scale water change. Slow and steady is the best way when treating old tank syndrome. Daily water changes of 10% for 5 days is a good start, and make sure to consistently check the water chemistry parameters. 

If the water changes haven’t lowered the nitrate levels sufficiently, then you can do the following. Even if the nitrate levels have been lowered enough, you should still do these steps, as the nitrates may increase again if you don’t. You can make use of nitrate removers. While there are limited options to directly remove nitrate on its own, there are things you can use. For instance Seachem DeNitrate is highly porous biological filter material which removes nitrate by allowing for anaerobic denitrifying bacteria to grow and proliferate within the structure. This denitrifying bacteria will remove nitrate. 

Dealing with the phosphate levels is extremely similar to dealing with the nitrate level. You can use chemical filtration media which are designed to remove phosphate from the water column as it runs through the aquarium filter. A great aquarium phosphate remover is Seachem PhosGuard.  

Seachem PhosGuard
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In order to replace the diminished carbonate concentration you can do a number of things. The KH is very important, and needs to be addressed. There are a number of buffering products that you can use to reintroduce carbonates into the water column. The water changes will introduce some back into the water, as tap water will contain trace minerals.


Preventing old tank syndrome is definitely preferential to having to treat the effects. It can be quite hard and a lot of work in order to get the water chemistry completely right. 

As you could have probably gathered from reading the rest of this article, the most important thing in preventing old tank syndrome is regular tank maintenance and cleaning. 

Weekly or bi-weekly water changes are essential in keeping nitrate and phosphate concentrations down, while at the same time reintroducing carbonate, bicarbonate, and other trace minerals such as calcium and magnesium. Weekly water changes of between 10-15% are recommended over slightly larger bi-weekly water changes of 20-25%. Using RODI water is recommended over treated tap water for water changes as all contaminants have been removed, reducing the compounds which cause old tank syndrome.

Organic debris sitting on the substrate will decompose and release phosphate, as well as ammonia which will be converted into nitrate as it goes through the nitrogen cycle. Therefore removing this debris is essential. As part of your weekly water change, you can use an aquarium vacuum cleaner or water changer such as the Python Water Changer to vacuum the debris as you remove water.

You should be properly cleaning and maintaining your aquarium filter monthly as part of the tank maintenance. Without monthly maintenance the mechanical filtration media will quickly become clogged. This will reduce the effectiveness of the filter greatly, and will also mean that as the waste that is trapped by the mechanical media pads decomposes, ammonia and phosphate will be released into the water. If you do not clean and change the biological filtration and chemical filtration media then their effectiveness will also greatly diminish, and in the case of chemical media may actually start to leach contaminants back into the water. 

Using an effective chemical filter is essential. A media such as will remove remove organic nitrogenous material which would otherwise be broken down to ammonia, nitrite, and nitrate.

You should also be careful to not overfeed your fish. Overfeeding directly harms your fish and causes them to be overweight and more susceptible to disease. Overfeeding will also cause more excess food waste. This excess organic waste will contribute to nitrate and phosphate levels increasing. 


Hopefully this article on old tank syndrome has informed you of what this aquarium condition actually is, the signs and symptoms you should be on the lookout for, the causes, and finally what you can do to treat and prevent old tank syndrome. 

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