the return section of the sump is where the water level fluctuates. the overflow/drain from the DT fills the sump. the water level in the sump is set by the baffle height. the only place where the water level will fluctuate is the return section of the sump. Like just about everone else, I have marked the levels on my sump in the return section as to where the water will go when i turn the pumps off and where the water level has to stay above to prevent the return pump from sucking air. When I do water changes, the fresh water goes into the DT as this is where the old water comes from. I then do my top off into the return section and bring the water up to its highest point.