second that ^

there really isn't such a thing as to much of a water change, unless you remove all of it and leave your fish lying in the gravel...that is too much.

I just did an 85% change thursday....and it actually was the best thing for my tank:
cleared up the water, helped get rid of my diatoms, and help with a baby spike in ammonia.
fish are happy, fine and enjoy the fresh water, as do my plants, so that wasn't close too much. I left about 3inches of water in the tank...just enough for all the fish to be able to get around still and get stressed by it.