David Baruchowitz, a reknown oscar keeper and editor of TFM, has advocated changing up to 90% of your water weekly. He has utilized a formula that shows that if you change only 30% of your water, you are leaving 70% of bad water behind. If this continues, you will never rid tank of all the bad water.
Of course, this is a drastic notion and flies in the face of the conventional amounts that most aquarists are used to. David did mention that this is not recommended for planted or immature tanks.
He actually demonstrated doing it in a large Oscar tank. There were at least 6 oscars in the tank. When the tank was left with about 10% in it, the Oscars were actually laying on their sides. When the new water went in, they immediately perked up and most, if not all of them stayed directly under the incoming water stream.
Pretty cool stuff.
Oh, I change 30% weekly in my 30
gal and once in awhile in my 75 planted.
