water changes

[fishguy007]

i have a 30 gallon aquarium. saltwater reef

are water changes needed? i have not been doing any as I am new to the hobby and the pet store told me that they were not needed. they told me just to top up the evaporated water with fresh water and i would be fine.

my question is are:
- water changes needed in my tank?\

- what will happen if i dont do them?

-and when should i be doing them and how much each time.