I would never do a 100% water change. You would never get all the levels like ph, temp, salinity, etc.. within range, and not have adverse reactions. Besides that, what logical benifit would you get from doing 100% rather than 50% water changes for say 2-4 days straight. If you want to nock out some high levels, do it over the span of a few days rather than minutes or hours. Patience plays a big factor in the up keep of aquariums.