Sump tank installation and why you should have one
The first question that comes to mind is "Why use a sump tank?" The most obvious answer is that more water volume is better. The larger the quantity of water circulating through the reef aquarium the closer it is to the real thing. There are many other good reasons for using this configuration as opposed to using just one tank. Among them are:
Hide tank equipment
It gives you somewhere to put all the equipment such as heater, skimmers etc which would normally clutter up your display tank
Add auto top off to your tank
It is a good way to add an auto top off system so that you don't have to keep topping it up manually check out DIY auto top off for information on creating your own top off system.
Allows you to add a refugium to the system for keeping otherwise unwanted or incompatible livestock, algae and plant material. These can help improve water quality and keep things such as nitrate levels at the correct level. See the nitrogen cycle for information about the nitrogen cycle and how this effects your water quality.
My sump setup
Just for information here is the sump tank I decided go with but this was only these specific dimensions because that is the size of cabinet I had to fit it in. This is another example of building your reef to suit your needs and requirements.