Marine Depot will not warranty the mag pumps if you run them externally. I ran a mag 18 on a Oceanic 75g for years externally. When I sold the tank the person who bought it from me replaced the impeller and had a issue with it leaking so I sealed it with a high grade silicone and it is still going. The mag 18 I had did not pump 1800 GPH, it was more like 800 at a 5' lift.