An aquarium sump is a significant component in an aquarium that is located beneath your display tank along with filter, heater and protein skimmer. It is a separate tanked that is plumbed into your main tank. They are of a phenomenal value in both in saltwater and freshwater aquariums since most of the best equipment required for the functioning of the aquarium is meant to be positioned in a sump. They are implemented in saltwater aquariums most of the time. A sump is usually placed inside the cabinet of an aquarium. Sometimes they are also placed in a separate basement or a fish room on a larger build which offers more accessibility. Most of the aquatic hobbyists set up the sump inside the cabinet and to move the water back to the display tank, they use a submersible return pump.
Sumps provide profuse advantages. They allow a large amount of extra space that can conceal your equipment, provide you a great filtration system, and it gives you flex space to be built that can be utilized as a secure place for plants in freshwater aquariums or macroalgae and fauna in saltwater aquariums.
The basic design of an aquarium sump consists of the following:
1. The water flows from the display tank located above towards the aquarium sump generally through a filter sock that removes debris or any large particles present in it.
2. Water is then skimmed to remove organics using foam fractionation when it moves through the chamber that holds a protein skimmer.
3. Water is biologically purified when it enters the refugium area.
4. The microbubbles are then trapped when the water passes through baffles.
5. Then the return pump forces the water back into the display tank.

Filter Sock
A filter sock is usually used as the first step of filtration to remove any particles present in the water. As the water flows from the display tank into the filter sock, suspended particles get trapped in it and eliminated from the water. The socks are then required to be replaced when they get dirty and clogged up. Filter sock is basically, your sump tank filter. A filter sock is a mesh or fabric material that covers the water pipe draining into the sump from the display tank and is responsible for removing large particles from the water before it circulates in the rest of the aquarium sump.
Protein Skimmer
A protein skimmer is a component of your aquarium that cleans the water through tiny scrubbing bubbles and gets rid of all the dirty particles present in the water. The water after passing through filter sock enters into the protein skimmer section and organics are skimmed using foam fractionation. After this, the water enters the next section of the sump. The protein skimmer can be installed anywhere in your sump but it is preferred to position it upstream the refugium because if placed after the refugium, it can suck up beneficial invertebrates that are being produced by the refugium and can skim them out.
Refugium
The refugium is that part of the sump that removes impurities from the water naturally. This is done by the organisms inhabiting the sump refugium area. Many fish hobbyists keep a deep sand bed in the aquarium sump refugium. This deep sand bed provides a growing area for beneficial bacteria and invertebrates. A typical sump refugium may also contain Chaetomorpha or other macroalgae that remove nitrates, silicates, phosphates, and other impurities naturally.
Baffles
Generally, after the sump refugium, water flows through a series of baffles. Baffles are actually, narrow walls to trap bubbles that force water flow sequentially over and under the walls. Bubbles are forced due to the quick flow of water under the first baffle. The bubbles are then trapped above the second baffle and then under the last baffle, the bubble-free water flows.
Return pump
The water is then sent up against the force of gravity back into the fish tank via a return pump.
Best Aquarium Sumps and Refugiums
There are many sumps and refugium designs in the market. It is quite overwhelming to make a choice from them and figure out which of them will work the best for you. Here are a few options for you to have a look:
Aqueon ProFlex Sump

Aqueon ProFlex Sumps are one of the flexible sumps for any setup. They are quantity made sumps that can convert depending on the type of running setup of an aquarium, into several setups. This means it can accommodate many setups for both freshwater and saltwater tanks. Aqueon provides a lifetime warranty on all of their sumps. It is a great choice for setting up sumps for the first time. The major disadvantage of this sump is that it has only one drain which does not allow to set up a bean animal overflow.
Leviathan Reef Sump

This black, white and red designed Leviathan Reef sump is one of the ultimate sumps that includes all the features. It contains 7 inches filter sock chamber along with dual drains and an adequate sized skimmer and refugium sections. The sump has a freshwater reservoir that can easily hold 5 gallons. It is highly durable with an attractive price.
Trigger Systems Ruby Sumps

Trigger Systems has been a recognized unit for its exceptional craftsmanship in the sump making industry for many years. The Trigger Systems Ruby Sump presents an ideal three-chambered sump. The refugium and skimmer sections are satisfactorily sized along with dual drains. The filter sock holders fix themselves into a place that avoids their accidental falling into the sump. It is a fairly accomplished design with good aesthetics.
Pro Clear Aquatic Systems Freedom Reef Sumps

The ProClear Aquatic Systems Freedom Reef Sumps are valued price sumps that are specifically designed for marine tanks. They come in unique colors with probe and sock holders, and a distinct bubble diffuser tubers that stop the noise of incoming water flow. They are ideal for the sumps that are protein skimmer driven as they do not have a refugium section. A 4 in 1 sump that is designed for freshwater systems is also sold by ProClear. It comes at a great price with flex hoses and dual drains for bean animal set up.