A sump is place where you can shove most of your filtering equipment. It allows you have more space for them so you can also do many types of filtration in it. Best of all, it gives your main tank a nice "natural" look without any pumps, heaters or filters in the view. If you use gravity to get water into the sump, you have to watch out for browns outs, you might flood your floor =).
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-Mark Mendoza
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