A sump is a 2nd vessel of water aside from your display tank that can serve a number of purposes.
- It adds to your overall water volume, helping to keep parameters more stable.
- It can be used to hide unsightly equipment such as heaters and skimmers
- It can be used as filtration, both mechanical, and biological (especially if you add a refugium to the sump)
- It works as a great place to setup other additional equipment, such as auto top off systems, autodosing systems, reactor vessels/pumps, monitoring equipment, etc.
You can use any container that holds water as a sump. A lot of folks will use rubbermaid containers as their sumps. Most folks will use a second aquarium. They are easy to setup yourself, but you can also find ready-to-go sumps such as from aqueon. It is pretty easy to install a sump yourself. You can make it as simple or complex as you like. Just some basic DIY skills are required to transform an empty tank into a great sump.