Old Tank Syndrome

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Old Tank Syndrome (OTS) is the term given to several conditions that can occur in a long-established but poorly-maintained aquarium.

Occupants die after a clean out[edit]

Owner decides to give the old tank an overdue cleaning and a large water change. The old tank has almost run out of buffering (KH has collapsed) and the pH is low. The result of the water change is that the new water has a higher pH and the free ammonia in the tank (nitrifying bacteria struggle to work in low pH levels) which was less toxic when the pH was low is now lethal as the pH rises. The animals are now suddenly exposed to the high levels of ammonia and changing pH swings overnight and die within a short time if the ammonia or low KH was not noticed and corrected for.


  • Don't be negligent about water changes and monitor the pH weekly for signs of it dropping.
  • Before doing a overdue cleaning, measure the total ammonia, nitrate, pH and KH levels first.
  • If there is ammonia add an ammonia removing chemical first. Perform only a small 10-25% water change, you don't want to pH shock your pets.

Buffering runs out[edit]

When the water runs out of buffering (KH levels are low (< 4 dKH) ) the pH swings rapidly during the night or day and can crash suddenly, often killing the occupants due to pH Shock.


Monitor the KH level of the water.

  • If you have soft water - Add some form of buffering agent like bicarbonate of soda if it seems to be getting low.
  • If you use medium to hard water - perform regular water changes to top up the KH.

Denitrifying bacteria grows[edit]

Ammonia levels increase as reverse nitrifying occurs. Deep in the substrate, denitrifying bacteria grows where there is no oxygen present and converts nitrogen back into ammonia.


Clean or turn over the substrate regularly. Ensure it is not too deep (< 2").

Nitrate poisoning[edit]

Another symptom is the owner has allowed the nitrate level in the tank to rise above 100 mg/l. While the aquatic animals living in the tank have adjusted to this extreme level, any newly introduced animals quickly die within a day due to this high concentration.


Monitor the nitrate levels and perform a 25% water change if too high.

  • If the level is found to be extremely high then the level must be reduced slowly as not to shock and harm the occupants. Perform a daily 10% water change in that case.