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I'm interested in building a sump for a new 37g tank using an extra 10g tank that I have. I've done some searches here and on reefaquariumguide.com. All the diagrams show the inlet to the sump labled as "Over flow from tank". What is this overflow? Is it another piece of equipment? Also, all the sumps show internal partitions. How does one know how tall the partitions need to be, how far away they should be placed or how high off the bottom of the sump they should be? Can anyone help in the hydraulics involved: flow rate, etc.

thanks.