Old tank syndrom is basically what happens to a neglected tank over time.
Nitrates rise steadily. A result of the nitrogen process hydrogen ions are released, and this process drives the ph down. A neglected tank will have a ph down to extremely low levels. So low, in fact, that ammonia is converted into a harmless form, much like the common additives do with it.
The fish adapt to this decline in conditions, and while by no means are they thriving, they will live.
If you were to suddenly decide that you want to bring the tank back up to spec the clean up must be gradual, or the change will shock the fish. Water changes must be done regularly in small sizes, the gravel would need some serious vacuuming, again a bit at a time. Restoring an aquarium with 'old tank syndrom' is a gradual and careful process.