They sell plugs for those holes so you could keep everything in tank. If you wanted to make a sump, I guess you could. Most people don't really mind the look their QTs as they are just that, QTs. Also, unless you already had everything (another tank, return pump, etc.) it would be a lot cheaper to just plug that one, or invest in a new 10G. Also, you may want to paint/cover 3 sides of the QT black or blue.