If you then refill the tank to the 100% level, your nitrate levels will be 20% of the original level that you started out at. If, on the other hand, you reduce the 40% water level once more to 20% and then refill the tank, you will end up with a nitrate level of 10% of what you started with. Perform the 40% to 20% reduction once more, and you will end up with a nitrate level of 5% of what you started with. Just think about it for a minute. If you started out with a nitrate level of 100 ppm and used this method, your 100 ppm nitrates would be reduced, in a short period of time, to 5 ppm, which is considered, by most, to be an acceptable level even for corals.