One should think, starting out with a new hobby is like learning by trial and error, or better, learning by doing. Unfortunately a certain amount of knowledge is required to start the hobby of fish keeping. The understanding of what is going on inside the tank, as you can't see, feel or smell pH, hardness, ammonia and such.
Pouring in one load of chemicals after the other and trusting this will solve problems overnight, will only end with the lasting memory of stores selling expensive products that do not work.
New Tank Syndrome, as the phrase implies, is related to a newly set-up aquarium. This may be the main obstacle the new hobbyist has to face.
In earlier references, New Tank Syndrome was often referred to as "mysterious" or "sudden" fish death. This definition has been re-defined as "The Cycle", as the knowledge about the hobby grew within recent years.
During the cycle, two sets of bacteria responsible for breaking down ammonia and nitrite are settling down. As these two compounds will spike to dangerous levels, the "mysterious" and "sudden fish deaths" starts making sense, as both compounds are highly toxic.
Ammonia, nitrite, and finally nitrates are nitrogen compounds, that accumulate during this stage and cause problems on a short and long term basis. The highest accumulation can be measured right around the time when the tank has cycled.
While in the past the cycle had been defined as new tank syndrome, today it is separated from the cycle by definition, but still causes a lot of hobbyists to throw in the towel.
New hobbyists often get excited when completing the "cycle". After all it has a scientific ring to it and it wasn't too difficult to achieve after all. The next step according to plan is to stock the tank with additional fish, as all levels read safe values. Once stocked, the problems take their course.
The reason is excess nutrients created by the newly introduced fish. The bacteria colonies are not able to readily adjust to the new circumstances, which would allow them to prevent the new tank syndrome. In return bacteria colonies start to grow, sometimes at a rate that significantly reduces oxygen availablity, besides not being prepared to remove the extra waste created.
Stocking the tank slowly can prevent new tank syndrome, giving the whole system enough time to adjust. Another way to avoid the new tank syndrome is by cycling the tank using the fishless cycling method. This method allows stocking the tank as soon as the cycle has been completed.