Just to make things confusing, in my last blog post I mentioned that New Tank Syndrome can happen to older tanks as well (when their beneficial bacterial populations get physically or chemically damaged), well with established tanks you can also get Old Tank Syndrome!
This is a phenomenon usually noticed by reef aquarium owners where sometimes the novelty has worn off and aquarium keeping looses its appeal (how can this be true!), which means than instead of staying right on top of all the water parameters, people can become a bit lazy and things start gradually slipping.
This Old Tank Syndrome manifests itself as corals just not growing as well as they used to or new marine life getting very stressed or dying when dealing with your display tanks water; your existing fish will be just surviving in the water as they have got used to the gradual negative changes over time.
So the cause of Old Tank Syndrome is a gradual drop in water quality over time due to neglect.
The primary things that are usually neglected are regular partial water changes and general maintenance and cleaning leading to an accumulation of undesirable chemicals in your closed system. Mainly this would be nitrate and phosphates increasing and pH slowly slipping down at the same time as the buffer ions are used up in the water.
Some aquariums have heavily buffered water and this situation is more dangerous as all the buffering ions gradually get used up by the hydrogen ions produced in the nitrogen cycle after all the buffer ions are used up the pH can crash fast which is dangerous for your marine life.
So basically you cannot underestimate the good that regular partial water changes and vacuuming up debris can do for your aquarium.
Saltwater Aquarium Advice