OK, so I know that water goes through the overflow box/drilled holes in the tank and into the prefilter. It then is sprayed or dripped into the bio-balls or other bio material in the bio tower. Then, it goes to the sump, which is what I don't understand. So the water in the sump is pumped back into the tank. Wouldn't it be pretty much impossible to get the waterflow going into the filter and the waterflow going out of the sump exactly the same? Even the slightest difference would cause problems a week or two down the road. Is the filter in the sump able to run dry part of the time? It seems to me that would be loud/cause problems with the filter. Can anyone ease my confusion?