how often to do a water change as always been a big debate, on here.
I have to agree to the most part with fishguy, if you have a well established tank and the right filtration equipment and you suppliment the tank with the elements it needs then you can go a lot longer without a water change.
on my old 80 gal. it went over 6 months before I took the tank down and started the 125. and I had no algae, 0 no2, 0 no3, cal. 430, all test showed the water quality to be great, everthing in the tank was doing great.
But when I transfered everthing over to the 125 with new water things looked and did a lot better, the corals went crazy, ones that were growing slower took off. It was a big difference. This tells me that yes I could go for 6 months maybe a year, who knows how long, but if I want to get the best out of my tank I need to change out some of the water at times, even though you add elements to suppliment whats lost you can't get the right balance back, the water change helps to put things back to were they need to be. I plan now on a 10% water change a month and I think I will have a much happier system.