I do 10-20% almost every day in a heavily planted tank with allegedly "touchy" fish (discus, rams, neons, a farlowella cat, and spectacular werneri rainbows) and my fish are very happy (rams in breeding color, rainbows in constant show-off mode, discus acting like regular fish instead of frail divas). It sounds like a hassle to do on a daily basis, but it only takes a few minutes. I treat my tapwater only with Prime, no other conditioners or buffers; but I do add several supplements for the plants (iron, trace elements, etc.). I had a scare the last time I did a 50% change (maybe my H2O was a bit "off" that day), but the fish seemed really stressed for several hours, so I figured that I'd rather stick with higher frequency/lower percentage changes.