Fish are supposed to swim in the display, not the filter. The AP12FS keeps them in their place by protecting them from jumping into the rear filtration chambers. As any one who has experienced this can tell you, it is very hard to get them back out and can hurt or kill a fish. Solid black in color blocks light from the rear of you tank. If you run a fuge this guard will keep you from seeing the light that escapes through the filtration gap.
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