10-06-2021, 21:27
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Old aquarium syndrome is a general condition in which the aquarium environment has deteriorated over time, especially the chemical composition of the water. With the exception of some cases of algae growth, there are usually few visible signs of significant changes that have occurred in the water parameters. However, water tests show a different picture.

Signs and symptoms

Nitrate and phosphate levels increase significantly. The pH, GH (total hardness) and kH (carbonate hardness) will be very different to fresh water. As a rule, the pH value will become more and more acidic with time. Decreasing pH value is often a sign of old aquarium syndrome.

Many inexperienced aquarists may assume that everything is fine because the fish are still alive, or at least most of them are. However, when new fish are added, they usually die within a short time. The reason for their death is often not known to novice aquarists. Some aquarium owners may suspect that something is wrong at this point in the aquarium and carry out a general cleaning. This usually results in more fish being killed. Why? Because the fish have been exposed to rapidly changing aquatic conditions.

Syndrome of the old aquarium


Understanding what causes old aquarium syndrome is as simple as realising that your aquarium is a closed environment. Like a new home, a new aquarium is clean and untouched. Just like in a house, everything becomes a mess when someone moves in, in this case the aquarium is populated by fish. Excess food and fish excrement settle at the bottom of the aquarium and accumulate in the substrate or water filter. The water from the aquarium gradually evaporates, leaving salt stains on the glass. In your home, you periodically clean the refrigerator, vacuum the floors and take out the rubbish. Who does this in an aquarium? If you don't, nobody does. Anything that gets into it stays in it in one form or another until you take steps to clean it. If the aquarium owner does not carry out regular maintenance, the tank gradually accumulates waste products which change the chemical composition of the water.

Although the filter removes most of the waste from the water, the waste still remains in the filter element until you clean it. The same goes for toxic chemicals like ammonia and nitrite. Yes, beneficial bacteria will convert them to a less toxic form. However, the by-products of this conversion process are other chemicals that are harmful to fish health in large quantities. Remember that everything stays in the aquarium until you clean it.

Since this whole process takes place slowly, the fish in the aquarium have time to adapt to the changes in water chemistry. The weaker ones often die, but the stronger ones survive, although their lifespan is considerably reduced. Any newly added fish may quickly die because they cannot adapt to the water chemistry.


Slow and steady are the key words for correcting old tank syndrome. Do not make abrupt mass water changes, as the aquarium may become clean but all the fish will die. Instead, perform a daily water change of 10-15% with fresh water. Monitor the ammonia and pH levels carefully, checking them daily at first. If the ammonia level rises quickly, skip water changes for a couple of days to allow things to stabilise. Check nitrate levels weekly to determine if they are dropping properly.

As the water parameters improve, wash the filter media in the water filter. Again, constantly test the water parameters to ensure that the water chemistry is at an acceptable level. The ultimate goal is to have zero ammonia, low nitrate levels and a pH close to the original water source, be it tap water or specially treated water.


Preventing old aquarium syndrome is a far better approach than neglecting aquarium maintenance and postponing preventive measures until later. Prophylactic maintenance and water parameter tests should be carried out regularly, not when a problem occurs. Water changes should be carried out weekly. The water filter should be cleaned every month along with the inside of the aquarium. Trash, such as excess food, should be removed from the aquarium as soon as it appears. However, cleaning is not sufficient. Water tests are the key to solving potential problems.

A monthly test cycle is sufficient if done conscientiously. Be sure to record your results so you can easily compare them with previous tests to see if a pattern appears.

If the pH changes, or if you see any of the other parameters changing significantly, you should change the water more frequently. With proper care and careful observation of the water parameters and the aquarium inhabitants, your little underwater world will always enjoy its beauty.

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