Here is some info about sumps that I copied from a website:
An aquarium sump is simply another tank of water that houses various forms of mechanical, chemical and biological filtration. The benefits of having a sump on your aquarium are as follow:
1) A sump will increase the overall water volume of the system, leading to a greater stability of water parameters (including PH, salinity, Carbonate Hardness and Calcium levels). The larger water volume will also be more forgiving if an inhabitant dies, a dangerous chemical is dropped into the tank, or if your chiller/heater breaks resulting in a temperature change. Basically, with larger water volume in an aquarium, changes occur more slowly, thus giving you time to remedy any problem before it becomes a serious issue.
2) A sump provides ample space for all three types of filtration methods. Personally, I have a 20 gallon sump tank (refugium) attached to my 65 gallon reef aquarium. The flow in set up in my reef tank so that the water from my overflow goes directly through my protein skimmer. The water then flows through the refugium section where it flows over a wall into the pump section. The pump section houses my Phosphate remover, my activated Carbon, and my Mag Drive 5 water pump. All these things fit out of sight under my aquarium and there is still space available to add additional equipment if needed.
Although there are noteworthy advantages to having a sump filter system on a reef aquarium, there are also a few disadvantages and limitations with this system. The disadvantages and limitations of having a sump filter on your aquarium are as follow:
1) A sump requires additional room. To house your sump filter, you will need either a large enough area under you aquarium to place it or a way to pipe it so that it could be placed elsewhere. I know plenty of people that choose to have their sumps to the side or even across the room from their reef aquarium, but the extra piping might be too much for the novice hobbyist to tackle.
2) A limitation to the sump system correlates to the size of the aquarium it will be attached to. For example, it would be unnecessary to place a 10 gallon sump on a nano reef that is less than 15 gallons. The nano tank will most likely come with some sort of internal filtering system that is more than sufficient for filtering the water. I have, however, seen people that have large reef systems piped in series with other smaller tanks. With this design, they have a large display tank with one or more frag tanks, refugiums, or breeder tanks all piped with the same water.
