Here we go again, LOL. The Myth of changing too much water, raises it's ugly head again. Ask any Discus or angel breeder how much water you should change and you will get an answer of much more than 60% per week. I have done 60% daily changes while growing out angels. I would have absolutley no qualms about changing 98% at a time if I thought it was necessary. The more often you change water, the more likely your tank water is to stay similar to your tap water, with regards to parameters. The ideal situation would be continous, flow through water change, like a trout farm. This of course, would be impractical, with home aquaria, hence the need to find a happy medium, that allows us to maintain our tanks, with a minimum of effort. That does not make it the best way. Bottom line is, that the bioload in the tank determines how much and how often water needs to be changed. However, that is the minimum,and doing more than the minimum is not a bad thing. You can't change too much water, too often. There may be exceptions to this, but at the moment, I can't think of one.