Doesn't really matter when you use it. I add 1/4 cup to a rubber maid container, add water to the container and cap it. I then place the container under the Magnum intake and take off the cap. The filter sucks in the powder and the powder gets lodged in the pleats of the filter.
If it slows the flow too much and your reactor doesn't get enough flow you can either run the filter at night when the CO2 is off or just cut off the lights and CO2 for the day and let the filter run until the lights come on in the morning. A partial day without lights won't hurt the plants and may actually help remove the greenwater quicker that way.
I may be down your way tonight so I may stop in and see how it's going
