What are self-priming externals, and why might I want one?
Externals work by drawing in water via a syphon. As such the inlet hose and canister need to be filled – and the hose must be syphoning – when you try to start up the filter or it will try to empty itself and run dry. Self-priming externals have gadgets, like plungers and refill-inlets, to allow you to fill the body up and start the filter. Without this, you’d need to suck on the inlet or the outlet to get the filter going. Filters without this feature are now considered a little old-fashioned.
