This myth is so widespread that even experienced reefers believe it. People that hold to this concept seem to believe water changes remove beneficial bacteria swimming in the water column, and thereby water changes remove the bacteria you want.
While it is true that some bacteria are carried about in the water column, for the most part, bacteria do not swim in the water. If we could count them we would have several million floating around in a small tank. However, bacteria carried in the water are only a tiny portion of the overall bacterial population of a tank. The bulk of the bacteria in any tank are sessile, meaning they are attached to solid objects in the tank. A single grain of sand may have a million bacteria growing on it but those lost to water changes are very small compared to the total numbers in the biological film.
In a new tank, we are adding live rock and possibly live sand that has this biological film covering it when it is received. It is all the population that the tank will need. If the rock is curing there is an abundance of dead and dying organisms on the rock. Most of the die-off consists of sponges and other organisms that didn’t survive the trip to your tank. As the rock cures beneficial ammonifying bacteria feed on the nitrogen released by decaying organic matter and multiply at astonishing rates. A single bacterial cell may become 64 in less than a single day. Since the bacteria that turn protein into ammonia reproduce faster than those that consume the ammonia and change it to nitrate, ammonia levels climb. Ammonia levels reach toxic levels in fairly short order and surviving higher organisms on the live rock die.
This is a time to for large, frequent water changes. The water change helps remove ammonia, and other decay products from the tank, and helps the tank achieve equilibrium in which ammonia can be processed to nitrate almost as soon as it forms. It lowers the numbers of organisms that will be lost on the rock as it cures. Sure, the water changes also remove some small number of beneficial bacteria from the tank but there are far more attached organisms on the live rock and sand than are needed in the first place. The lost bacteria have no bearing on the time cycle will take and the following algae outbreaks will be shorter in duration as their will be fewer nutrients in the water column to help fuel the algae growth.
Do yourself and your new live rock a favor and do at least 25% water changes every day on a cycling tank. Even larger water changes are perfectly fine and probably will shorten, not lengthen, the cycle period.

__________________

I agree that the myth is a horrid one and the thing that got it started was the practice of some new hobbyists doing 100% water changes in their tanks. In no way should the water changes exceed 50% during the cycle but they should be done for sure to control the amount of ammonia build up. It is different if you are doing a fishless cycle where the tank is not containing fish to be concerned with. Then they may be desirable but not absolutely necessary. But when there are fish in the tank assisting with the cycle the ammonia and nitrites and nitrates need to be controlled with water changes.

thank's for you reply

__________________