Another consideration you need to make when setting up your sump is, "What will happen in the event of a power outage?"
When you shut you return pump off, water from the main tank will flow down to the sump. You need to maintain a 'safe' area in you sump so you don't have a flood. To determine the water volume the formula is (L" x W" x H") / 231 = volume in gallons. (make sure you're measuring the inside dimensions of the tank)
Figure out the minimum "safe zone" needed in the sump by Calculating the volume of water between the water in the display tanks normal operating level, and the minimum overflow point. Then add consideration for any additional water that will drain out from the plumbing
Here's an example. My tank is a 55 gallon, my sump was created from a 20 gallon long. There is 1/2" between normal operating level, and the minimum level before the overflow stops. There is 3.5" between the top of my baffles and the top of my sump (measured to the bottom of the trim, not to the actual top of the tank). Now lets look at the math.
Tank overflow (12.00 x 47.50 x 0.50) / 231 = 1.24
If we add 1/2 gallon of water for each the overflow pipe, and the return pipe, that leaves the necessary safe zone at 2.24 gallons.
Sump "safe zone" (11.81 x 23.67 x 3.50) / 231 = 4.23
The actual "safe zone" (4.24 gallons) is nearly twice the necessary capacity (2.24 gallons). So in the event of a power outage, no flood.
