I am the extreme when it comes to changing water...I also think that this is one of the most asked questons in the hobby.
My answer: depends on the bioload. Let the tank and some common sense determine how much should be changed.
If your costantly moving things around,stiirring up the sand adding new livestock cosistantly Feeding Heavly then of course more water should be changed shy of a low bioload of livestock. Also another factor, your numbers. Cal. mag. alk. most of these can be maintained with consistant water changes and the use of good salt.Yes there r exceptions.
I am sure some will diisagree. which is all good...We Agree to disagree. This s just my personal opinion and no where near fact.
Oh yea i change anywhere fro 30-50 gallons a week out of my 210. This is the result.
[video=youtube;dEBu_ncu8qU]http://www.youtube.com/watch?v=dEBu_ncu8qU[/video]