they work great below the water, if they are too near the surface they tend to suck in air which make them noisy. They are good for moving a body of water across the aquarium, bang on the opposite wall, then back to receiver ( a bit like a conveyor belt). You can set them up horizontal or vertical depending on your landscape.
A closed loop system means that you would have to drill multiple holes in the bottom of your tank and a circulation pump will suck in water from one hole and push it out another hole. It efficient for creating flow in all space across all hard to reach crevices and can be easily hidden to almost invisible.
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