I've had the best luck with weekly or bi-weekly small water changes. HOWEVER, I think a lot of this has to do with the parameters of the make up water. For example, if the alk level in your make up water is 12dkh and your tank is 7dkh, you're most likely going to cause an alk spike if you're doing a 30% water change (i'm too lazy to do the math). I think that this may not be an issue if you don't have an SPS tank but your corals probably still won't like it much.