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Old tank syndrome occurs in fish aquariums with high levels of ammonia and nitrite and low levels of water pH. It can be caused by overstocking, but is most commonly the result of inattentive tank maintenance. This condition can affect an age or species of fish, but is most dangerous to new fish that are added to established aquariums.
The primary symptom of old tank syndrome is the death of new fish that are placed into a long established tank, while the old fish remain alive and apparently healthy. This is because the old fish are accustomed to the balance of the water, even adjusting to conditions such as build-ups of certain chemical or bacterial levels. The old fish often do not show any signs of being affected by the unhealthy levels in the water. The new fish, however, have been accustomed to a different water balance and are shocked by the sudden change in conditions.
On testing, the water will show measurable nitrite and ammonia levels, which can be toxic to fish, and a lowered pH level. pH levels below 6 indicates a serious imbalance, often leading to the loss of beneficial bacteria, which then leads to a dangerous and toxic increase in ammonia and nitrite levels in the water.
The cause of the high levels of ammonia — which leads to old tank syndrome — is often due to less than ideal water maintenance, and a sudden drop in the water's pH level. When the pH of the water suddenly drops below 6.0, the biofiltration system is unable to metabolize ammonia properly. This can also potentially occur when new water is added to a tank in excessive quantities.
If your fish are suffering from old tank syndrome, begin by adding a few gallons of new water each day. This will allow the water to adjust to healthy bacterial levels again, and the fish to adjust to the change gradually. Remember that your old fish have become accustomed to the levels in the water, even though the levels are unhealthy. Too much of a change to very clean water may kill your fish.
Once the beneficial bacteria are well established again, ammonia and nitrate levels will drop back down to levels closer to zero — as they should be. Never dump the water entirely and start with new water and materials, as this could result in "new tank syndrome," a toxic condition that can result in the deaths of all of your fish.
To prevent old tank syndrome, maintenance is the primary concern. New water should be added to the old on a regular basis to maintain acceptable pH levels. Never remove and replace the water entirely, as that could cause another set of problems. Additionally, testing the water balance is an essential part of caring for fish. Performing regular pH tests on the water will enable you to monitor and track the health of your fish water and make adjustments accordingly.
Ammonia levels greater than 2 mg per liter will cause toxicity symptoms in the fish.