Seems like a lot of your posts are asking the same thing, and we can't really offer advice until we get specific info on the following:
ammonia/nitrites (I assume 0.0)?
salt mix used?
water or tap water used to mix salt?
Starting with numbers/answers for all those things, then it should point to answers like how often to do water changes, if you need to dose anything in the tank for optimum coral growth, etc.
Without knowing those things though, I'd say with a new tank you should be doing a minimum of 20% water changes every two weeks. Personally, 10% changes every week have worked fine for me, but I'm not as stocked as your tank is. If we're talking about your 120g... there's a fair amount of fish in there, so more frequent changes might be in order depending on your nitrate readings.