I'm not sure I understand how these DIY pond vacs work. I'm referencing something like this http://www.members.shaw.ca/floydn/
The water comes into the bottom of the barrel and fills it up then you pump the water out of the barrel?? How is this filtering the water to put it back into the pond? Or is the water not be replaced? I'm getting ready to clean a large pond and wanted to explore my options rather than totally draining and refilling. Thanks.