How big is your tank? I am in the same predicament as you. I plan to convert my 135g in stages, a third at a time over a period of of about a month; as a result, by only removing a third of the cc you will not be setting yourself up for a potential crash. This approach is more of a drawn out drama, but seems to be safer. I like the idea of placing the sand in via the tube. Nofear you might want to look up some of my posts in the reef section regarding this very same topic. teetee.