I'm not sure. In cars, closed loop is a mode of computer operation where the computer is reading air temp. readings, o2 sensor readings, coolant temp. and MAP readings in order to properly mix the air and fuel for combustion...
In a fish tank it would probably be something like a power head that simply stirrs water around without sending it anywhere or maybe it's a term to describe the system as a whole. If you have your fish tank drawing water in from the ocean and pumping it back out again, that could be open loop.
If you simply recirculate water within the system and don't let it exhaust anywhere then it could be called closed loop. If that's the case then we probably all have closed loop systems.