Old Tank Syndrome: Signs, Causes, Treatment, and Prevention

Old tank syndrome can happen to any tank. Just like new tank syndrome doesn’t occur only in newly set tanks, old tank syndrome isn’t reserved for aged aquariums.

Old tank syndrome refers to poor water quality rather than actual aquarium age. It can and likely will appear in any unattended tank. The main signs of old tank syndrome are:

  • Increased ammonia and nitrates
  • Low pH (below 6.5)
  • Low KH (below 1.0)
  • Dying new fish

The latter of these signs is the most obvious and the most painful. Due to the slow water deterioration, older livestock may have had the time to adapt to the suboptimal conditions. Newly bought fish will likely suffer and die, regardless of how well you acclimate them.

But let’s examine in detail what causes old tank syndrome before discussing each symptom individually.

Old Tank Syndrome Causes

Old tank syndrome happens when the water quality in the tank deteriorates to unhealthy levels. Unhealthy levels mean low pH and KH, plus persistently high concentrations of ammonia and nitrates.

Several things can lead to drastic drops in water quality, but the most common causes are:

  • Regular overfeeding
  • Lack of regular maintenance
  • Mass plant death
  • Overstocking

Each cause leads to ammonia build-up, but they reach the same result in different ways.


Among new fishkeepers, overfeeding is probably the most common cause for poor water quality. Overfeeding has a direct impact on the fish, making them fatter than they should be. The invisible impact of overfeeding is more insidious, as the decaying uneaten food releases ammonia and pollutes the water.

Fish won’t die directly from overfeeding but excessive food will eventually cause death.

Poor Maintenance

Another very common cause for old tank syndrome is the lack of regular (or adequate) maintenance. This tends to happen to older aquariums when fishkeepers lose interest in the hobby (or life happens), which probably led to the coinage of the “old tank syndrome” phrase.

See, by default, our aquariums are closed systems. The waste doesn’t have many places to go and builds over time. Water changes and gravel siphoning help replenish the water column with minerals while removing debris and decaying waste.

Relying solely on the filtration system — especially if it isn’t cleaned every once in a while — is unwise. Even in a large canister filter, which technically stays out of the tank but in reality is a part of it, the waste builds up. The beneficial bacteria living in filter media have their limits and pace. They cannot process infinite amounts of ammonia in an instant.

Having some mulm and fish waste is perfectly normal but letting them accumulate is dangerous. Indeed, it is possible to set up tanks without mechanical filtration that require minimal maintenance, but these setups rely on plants and deep substrate to do the filtration. And they also need regular water changes, especially early on.

Inadequate maintenance leads to elevated ammonia and nitrate levels. The change happens over time and many fish could adapt to the lowering water quality. However, when you buy new fish and they die in a couple of days the problem becomes apparent.

Mass Plant Death

If many plants die in a short period, their decaying leaves will release a lot of ammonia. Now, plants dying en masse is a problem in itself and it may or may not be related to fish health. Plants can experience various deficiencies that don’t bother fish or other livestock, but the point is to clear out decaying leaves.

A dying leaf here and there won’t affect the tank too much, but a small forest of sick leaves will.


Overstocking is bad on many levels.

Too many fish will produce too much waste for the filtration system. In turn, you guessed it, this will increase the ammonia concentration and will likely kill fish.

Overstocking elevates the levels of stress, too, making fish more susceptible to various ailments and poor water quality. It can lead to aggressive behavior as well.

How to Treat Old Tank Syndrome

Once you’ve detected the root cause for old tank syndrome, dealing with it is relatively easy.

If you are overfeeding, reduce the amount of food. Feed them only as much as they can consume in 2-3 minutes. Fish can last days without eating, so don’t feel guilty when ignoring their pleas for more food.

The same goes for overstocking — keep fewer fish. If you think you have too many, sell or exchange a few in your local fish store. Of course, you can simply put them in a new tank, but that requires some time, as new tank syndrome also can kill your fish.

If many plants are dying, try to asses what causes their suffering and clear out the dead plant matter regularly.

Which brings us to maintenance.

Regular maintenance will address the symptoms of old tank syndrome. The key here is regular. Removing 50% of the water at once will likely make things worse, as it can change the pH too drastically. The process of ammonia breakdown consumes pH, which is why very low pH is one of the telltale signs of old tank syndrome.

By adding a lot of new water to the tank the pH can jump, leading to a serious pH shock.

Instead, perform several small water changes (with siphoning) over the course of 7-10 days. 15%-20% daily water change will lower the nitrate and ammonia levels, allowing the fish to adjust to their new environment over time.

By the end of the process, the water in the tank should have a similar pH to the source water.

Clean the filter media, too. Rinse it with the dirty aquarium water but don’t go overboard, as too thorough cleaning can kill too many beneficial bacteria and that can cause an ammonia spike.

Prevention Is the Best Cure

Prevention is the name of the game here. Maintaining your aquarium healthy is much better than trying to fix an underlying problem.

For starters, do regular tests. Strips are much faster but less reliable than liquid testers. I use them both. Tests with reagents are great for precise measures after a big water change or when a new fertilizer is introduced.

Test strips are perfect for weekly testing as they give you a baseline. While not extremely precise, they give a fairly good idea of what the pH, ammonia, and nitrate levels are.

Create a regular maintenance routine and follow it. It doesn’t have to be every week. Base the frequency around your test results. If the ammonia levels remain low over two or even three weeks, do water changes every two or three weeks. If such is the case, the water change may need to be larger, at around 35%-45%.

Again, test and see how a smaller water change affects water chemistry.

Clean dead leaves, don’t overfeed, definitely don’t put too many fish in the tank, and clean the aquarium regularly.

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