A sump, when related to an aquarium, is basically just a secondary tank positioned somewhere below the main tank and that is fed with water by way of gravity. The water is returned to the main tank with a pump once it has been processed in the sump. Generally, the volume of the main tank will pass through the sump a couple of times an hour. The sump itself can be configured in a number of different ways to provide specific functions that benefit the main tank in some way.
First and foremost a sump, even in it’s simplest form, adds volume to the system. If the main tank is 100 gallons and you add a 50 gallon sump, well then the volume of the entire system goes up to 150 gallons. With that added volume comes added stability. A larger volume of water takes longer to change in temperature, salinity, or whatever parameter you want to use. And as I’ve said over and over, stability is key to a healthy aquarium.
After adding volume, the next most common reason to include a sump in your aquarium setup is to give you a place to put all the equipment that runs the thing. Filters, heaters, skimmers- it can all go in the sump. This means less clutter in the tank or hanging off the back of it. Even more so it may be the only option if the back of the tank fills up and you still have equipment that needs to be hooked up. Furthermore, since the sump is likely located in the enclosed stand the noise all that equipment generates will be reduced as well.
All sumps are fed by some sort of overflow mechanism either hanging on the back of or built into the tank. This mechanism is built in such a way as to let the water from the tank spill over into it when it gets too high and flow down to the sump. The benefit of this is that the surface of the water in the tank is continually skimmed clean. Tanks without an overflow often have a greasy film of proteins and oils floating on the surface of the water which is problematic as it can block gas exchange. With an overflow, this layer is pulled into the sump and churned back into the water for the protein skimmer to handle. Additionally, that churning also helps increase gas exchange- increasing the dissolved oxygen level of the water.
A sump also means a more stable water level in the main tank. Marine aquariums in particular lose a lot of water to evaporation. On setups without a sump the water level in the tank drops as water evaporates, possibly exposing intakes or other equipment in the tank (or even corals that have grown very tall) to the air. And of course even if everything is low enough to not be affected you still end up seeing the low water level from the outside frequently which, while not exactly a tragedy, isn’t pretty either.
Possibly the best benefit of a sump that’s not immediately recognizable is that it gives you a safe place to introduce additives to the tank. Reef tanks typically need daily doses of calcium, alkalinity, and/or other supplements to keep the water’s parameters in check. Many of these chemicals are highly concentrated and if added directly to the tank need to be added very slowly. Having a sump where you can just dump them in to be diluted down before they enter the tank makes adding them much less of a headache. Likewise topping off evaporation is easier with a sump for the same reason. Relatedly, a sump makes a great place for the heater and/or chiller since the localized hot/cold spots they produce will be safely away from the inhabitants of the tank.
Finally a sump makes it possible for you to more easily make use of two techniques to improve your aquarium. The first is a trickle filter. Basically, as the water enters the sump it’s allowed to spread out and trickle over a filter media used to cultivate nitrifying bacteria. Since the media isn’t fully submerged the bacteria growing on and in it receives much more oxygen and is therefore able to perform much better.
The second setup a sump makes easier is a refugium. A refugium is essentially, as the name implies, a small secondary tank that serves as a refuge for algae and various microorganisms away from the hungry mouths within the main tank. The conditions in a refugium are ideally perfect for algae, which keeps it growing there instead of the main tank, as well as many planktonic creatures which fish and corals love to eat. And as the population of these critters increases more and more of them will start spilling over to the main tank to provide a supplemental food source. But since the main population remains in the refugium the fish won’t be able to completely destroy it.
A sump does add a bit of complexity to your aquarium setup, but with proper planning and design it really doesn’t add any addition work to your daily or weekly routine. And as you can see the benefits are many. Even just a simple no frills sump will greatly benefit your tank with the added volume, agitation, skimmed surface it provides.