Plumbing and The sump
Many manufactured tanks come “reef ready.” or you can D.I.Y. it all yourself . Drilled holes in the tank and an overflow box with Durso standpipe allows only surface water to be drained to the sump usually set below the main tank. Narrow slots at the top of the overflow prevent large objects from entering the plumbing. In some designs, the return pipe passes through a second hole in the tank bottom and extends up to the top of the prefilter, paralleling the standpipe. In other designs, the return pipe passes through the tank bottom at some distance from the drain. In still others, a return hose simply loops over the rim of the tank to discharge water just below the surface. A plastic part known as a bulkhead fitting provides a waterproof seal around the holes in the tank bottom. It consists of three components: a threaded pipe with a flange on one end, a ring nut, and a gasket. The pipe goes through the hole with the flange inside the tank. The gasket slips over the threads, followed by the ring nut. Hand tighten the ring nut until the gasket is slightly compressed against the tank. The inside of the flange has a female thread to which a standpipe or return pipe can be connected using a threaded adapter. Under the tank, the portion of the fitting extending past the ring nut has a male pipe thread to which additional pipes or hoses can be connected to transport water. The sump tank is placed underneath the display aquarium. Water from the main tank passes out the overflow and into the sump.
If the sump is part of a wet/dry filter, a separate box of filter medium sits on top of the sump. Water from the drain line passes over the medium before it reaches the sump





Some benefits to having a sump:

Increased total water volume – The larger amount of water you have, the more stable your water parameters will be. 
Increased Flow – The return pump in the sump will be more water flow in  the main tank .  
Hiding Equipment  – All of the equipment such as protein skimmers, heaters, and media reactors etc. can now be out of your display tank and hidden in the sump..
Maintaining your water level – As water evaporates from your set-up, the water level in the display tank will not change.  The reduced water level will occur in the  sump allowing you to top up for evaporation without affecting the main tank. An auto top-off system to control and maintain a consistent water level and salinity can be used.
Stabilizing your PH – If you have a refugium or live rock compartment with some algae, you can set up sump lighting to come on while the main tank lights are of, this allows for photosynthesis to continue in the sump. the main lights are outit stops in the main tank which controls the CO2 levels that typically affect the PH when all.

Increased Oxygenation – As water from the main tank enters into the drain line, air is pulled into the drain line with the water and mixes together, adding more oxygen and releasing some CO2.

Food Source for the display tank – Sumps can be set-up to provide a safe place for pods and other little forms of marine life to grow without the threat of predators.  These little life forms will find their way to the return pump and sent off into the display tank were they will become a food source for fish, inverts and corals.

 

Some drawbacksto having a sump:  ?

 

Large high flow pumps can result in placing micro bubbles in your main tank.
More equipment added to your overall set-up can add extra sources of heat into your water.
Larger return pumps do consume extra energy.
Sumps inside a stand can give you a very cramped area to work in.
Using some siphon based drains for your sump can leave you at risk of the drain stopping should you lose power.

If you have decided you want a sump after carefully considering the pros and cons, you are going to want to plan the sump as best you can.

 

 

 

As for plumbing the sump to your tank, the below article here contains a lot of helpful information on how to plumb a sump and tank together.

 

http://reefaquarium.com/2012/aquarium-plumbing-basics/