That's a gravel vac. You hook up the air hose and put the free end in a bucket then pump the vac end up and down in the water to get the syphon going. It takes some practice but its very useful if you have smaller gravel. I have two but I prefer a turkey baster in my smaller tanks. Another thing I like about underwater filters besides the lack of waterfall noise is that you can have a power outage or unplug it for maintenence without killing the bacteria. With hanging filters if the power goes out then the bacteria gets exposed to air and killed off.