Nice tank and sump. I personally do regular water changes and have a skimmer but I'm a relative newby to SW so I'm not chancing anything lol. Where does your Magnesium and to a lesser extent Calcium (I know this will be provided by the sand) come from if you don't do water changes?
it's now 105 days..... if there were any signs of distress I would not hesitate to do a change... Not one loss, no illness, masses of procreation, feather dusters, snails worms, strange eggs, funny things at night under the light, don't feed the corals, had to move one because of a migrating anemone, but thats about it.
Oops I was wrong.... I put in my new sump on the 25th of May, I counted 105 days from this post which was 1st of July, so in fact it's another 55 days since that, so my last water change was about 160 days...... no nitrate, no skimmer... no additives... I actually don't do much at all... hmmm.