It depends on what you are trying to accomplish and just comes down to simple math. If your goal is to reduce nitrate and say your nitrate is 40, a 20% water change only gets you to 32. You would need to do very frequent water changes at that volume to have a meaningful impact. There are other better ways to reduce nitrate load.
If your purpose is to replace trace elements for coral health then maybe 20%???? But even for this purpose it’s just guess work without advanced testing.
I still do water changes periodically despite my above comments. It’s an excuse to vacuum up any Cyanobacteria lurking in crevices, clean up the sand, give a good flush of the dead zones, clean the sump and inspect all my gadgets
If your purpose is to replace trace elements for coral health then maybe 20%???? But even for this purpose it’s just guess work without advanced testing.
I still do water changes periodically despite my above comments. It’s an excuse to vacuum up any Cyanobacteria lurking in crevices, clean up the sand, give a good flush of the dead zones, clean the sump and inspect all my gadgets