If your tank is properly cycled, a 100% change should not harm it. After all, there is very little BB in the water. As long as your filter remains in a bit of dirty water whilst you do the change, there is no harm whatsoever in doing a 100% change in a cycled tank.I never do a 100% water change. Destroys my cycled tank. I change 1 gallon a week out of my 5 gallon.