[mike31154]

Tunze is a company that primarily manufactures powerheads and wave boxes for water movement in marine tanks. I think they also make skimmers and a few other things. Anyhow, a Tunze powerhead is placed in your tank for water movement and can be situated pretty much wherever you want to create the flow you need. No holes required in your tank to use them or any other powerhead such as a VorTech or Koralia.

A closed loop is essentially a plumbing network which draws water from one area of your tank through a bulkhead (requiring a hole or multiple holes through the glass) and delivering it via an external pump back into the tank for flow, either over the top or through more bulkheads through the glass in several other locations. It can be fairly simple or get quite complex, depending on your flow requirements. Generally once a closed loop is set up, there's little you can do without major rework if your requirements change. A closed loop is separate and independent of your overflow, sump and return pump if so equipped. A conventional powerhead is easily removed and relocated although more unsightly in the display. A closed loop can be virtually hidden if done properly, although multiple outlets are required to create adequate flow in all areas of the tank since the openings are typically small with concentrated flow. This is where a Tunze or VorTech powerhead have a bit of an advantage since the flow is more broad and they are easily repositioned if required.

Lots of debate re closed loops vs powerheads these days. Each has its advantage & disadvantage. In the end I think it boils down to personal preference, since adequate flow can be provided by either method if implemented properly.