I did a 100% water change once, solved my nitrate buildup pretty lickedy-split
But it was a 20g tank and it had no fish, only a carpet anemone and some corals. But they all liked the outcome ...
There are times when I look at my 110g cube tank now with its elevated nitrates and I wish I could do a 100% changeout, I just don't have enough reservoirs to do it (and I have fish in there so that's no good). But I'd love to be able to change out even 50% or 75% of it just to be a kick-in-the-melon to the nitrates that are there.
Other than the bad salt (Kent, about a year ago? Maybe 2 years ago now, when they had that alk issue??), I've never had an issue from the waterchange itself. But yeah, I do match the temp and SG carefully, and do the premix for 24 hours thing too, so that probably helps a bit. I'd hate to see the results of a large scale water change that introduced a drastic temperature change.
I tell you one thing though, and maybe it's worse for me than others since I have tanks that have at least 5 years of crud buildup in the plumbing, when I turn the pumps off and turn them back on, the stuff that comes flying out of the sump return outputs is something I wonder if it would be better off not having been disturbed and released "en masse" into the water (ie, did I just invalidate my water change). I need a way to scrape out the pipes
Or maybe it's better just to redo the plumbing every few years or so
