06-08-2007, 08:21 PM
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To answer your question a bit more thoroughly.
Yes, that black contraption you see in the tanks at your LFS is the overflow. That, along with the holes drilled for the plumbing, are the defining factors of a "Reef Ready" tank.
A reef ready tank has the built in overflow, and two (or more) holes drilled in the tank for plumbing. This feature is most commonly applied to reef tanks where larger equipment such as multiple powerheads and skimmers are needed. The plumbing allows for water to be drained to a secondary tank called a sump, where the skimmer is housed. The sump also allows for heaters, which can burn corals and other inverts, to be housed seperately. the water is then pumped back to the display tank via a return pump. The output from the return pump also takes the place of 1 larger power head.
The alternative to a reef ready tank is the use of an external overflow. This is a bit less reliable because it works on the syphon principle. in the event that air enters the syphon tube, and the syphon breaks in the overflow, water would cease to flow to the sump. the return pump would continue to pump water to the display tank until the return section of the sump empties. This generally results in one or both of the following:
The display tank overflows onto the floor.
The return pump burns out from running dry, potentially causing a major fire.