I do 5gal changes on my 29gal reef every other day, which equates to about 60% weekly (60% of DT volume, 30% of total). It's horribly overstocked and a MUST! Nitrates would be over 100ppm otherwise. Also, I have to do water changes on my QT tanks, so I'm getting in the habit of doing the changes (just like working out). I haven't done any water changes on my 125gal, as it's just corals, so I just supplement as need. When I add the fish I will do weekly 20gal changes on that.