Because the surroundings like aquarium gravel in a fish tank are so vital to the survival of fish, even the slightest change could endanger the life of a fish.
There are some people who, after having a certain kind of gravel for a while, begin to consider changing the gravel for a different kind. This can be dangerous for the fish who live in the tank because a key component of a healthy aquarium is being able to maintain the beneficial bacteria that has been built up from the gravel.
One of the concerns that some people have is that most of the bacteria is found in the gravel bed, therefore if the gravel does not absolutely need to be replaced, it should not be bothered. Also, there are others who have concluded that most of the beneficial bacteria is found in the filter and not the gravel. In this case it would be just as easy to change the gravel and probably safe, too.
If you feel confident about changing your aquarium gravel, you should do so with caution and remember that the life of your fish depends on your confidence in the gravel changing experiment.