no problem with only turning on at night.
kh/alk is the most important of the three measurements both in terms of coral growth and tank stability, not calcium like most people think. check and make sure that is about right. ca is okay, maybe a bit low (most people aim for between 400-450), but mg is really low. from what i've read, mg should be ideally around 1300, but most saltwater mixes have from 1100-1200. below 900-1000 is low enough to potentially effect ability to keep alk stable..