you want your sump to be apx. 1/4-1/3 the size of your display. so depending on the total volume of the 3 tanks, a 10g sump, may not be big enough. if you cant get ahold of another larger tank, you can also use a large plastic rubbermaid tub. (20-25g tub would be fine and cheap too)

you can make 2 overflows (one for each of the outside tanks) and make the return (spraybar?) go to the middle tank. you can either run both of the overflows to one larger pipe, or just have 2 pipes going to your sump. either way will work fine.

if your overflows are built as directed in the diagram, and your sump/ overflows/ return pump are appropriately sized, you will not have to worry about either the sump or the tank overflowing.

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why buy, when you can DIY