I would be wary of running 18% peroxide the reason being once the water reaches a certain cleanliness the peroxide looks for something else to oxidize (best way I can describe it) if you have nems they can react negatively to high % of peroxide, yes I have been there in my experiments. You might find 18% fine at first but one day it might just bite you. The idea of an Oxydator is to convert the vast majority of the peroxide down to it's component parts, water and what the manufacturers tell super reactivated oxygen. With very high % of peroxide there is also a chance much of it might not be briken down and escape into the tank to burn sensitive animals like nems etc.