Here's a brief description of the very minimum equipment required for a sump to function using an external hang on back of the tank overflow. This is the "bare bones" required.
1) The overflow itself consisting of inner box with teeth, outerbox, U tube siphon and prefilter sponge.
2) A hose or pvc piping to connect the discharge of the overflow down to the sump.
3) The sump itself. This can be as simple as a Rubbermaid container or empty glass aquarium.
4) A return pump to pump water from the sump back up to the main tank.
5) A spray bar. Typically a group of pvc elbows or fittings that allow it to hang on the back of the main tank, that direct the return water back into the tank, and not allowing the water to shoot alll over the wall and floor of your home.
6) A return hose or pvc piping to connect the return pump to the spray bar.
7) Hose clamps if using hoses.
8) Some method to attach the hoses and pipe system to the sump so that they are in a secure fixed position, not just hangin there flopping around.
That's it.
The sump size depends on a couple things.
First off you never run it entirely full of water. 50% is a rule of thumb for the following reason. When you lose power or shut off the return pump, water will natually begin to drain back down into the sump from the main tank. To what level this continues will depend upon the depth you have the spray bar located into the top of the tank. If you have the spray bar all the way down to the bottom of the tank, you will drain the water in the tank down to that level.
NOT GOOD.
Most folks mount the spray bar an inch or so depth into the tank, thus when the power goes off, you will only siphon/drain back that depth of water to the sump. Thus you must chose a sump that will hold this amount of additional water, without overflowing itself. A small hole drilled in the spray bar just below the normal operating level of the main tank will prevent much water from draining back to the sump. It' another added measure of insurance to take advantage of.
Bigger is better with sumps as well. Adding additional water to your entire system is normally a good thing too for tank water quality and stability.
If you chose to DIY, most people get pretty creative, and make dividers seperating the sump into various chambers. Each chamber for a different reason. Skimmers, Heaters, adding chemical filtration like activated carbon in a mesh bag, possibly a refugium area and maybe a divider to reduce bubbles from entering the return pump.
Manufacturered store bought sumps normally have some or all of these chambers already designed into them. They are naturally more expensive, but look an function great. If you are not a DIY sort of person, store bought units may be the route to go.
Me ..... well the UPS dude just delivered my new LifeReef external overflow an hour ago.
It's a monster !!!
I've already torn into the box, and must go sweep up the styrofoam packing peanuts before the dog get's a mouthfull.
Then .... off to the Depot for some more pvc fittings
See ya !