Basic Filtration for a Fish Tank

The two most common types of filters, besides the canister types, are the wet/dry, for a Fish Only tank, and the sock sump, for a Coral Reef tank.  These are very different for the application that they are intended and should be selected based on the type of tank you intend to set up now, or in the future.

Wet/Dry Filtration

The way a wet/dry filter works is that water is brought directly from the aquarium via an overflow box, over a large surface  area  where  it  is  then  dripped  upon  (bio-balls)  to  promote  aerobic  bacterial  growth.  The aerobic bacteria cultivated then process waste within the system to create nitrates that can be easily removed during water changes.
This is a very effective form of biological filtration that should only be used with fish-only aquariums. This system can become problematic if large amounts of debris or particulate matter get trapped in the bio media chamber, creating what is commonly called "a nitrate factory." Because elevated nitrate levels are hazardous to coral and invertebrates, we do not recommend wet/dry filters for reef aquarium systems.

Sock Sump Filtration

The sock sump is used as a strictly mechanical filter, not biological, and must be thoroughly cleaned every few days. Every time you clean the sock you remove organic matter that would ultimately raise nitrates and phosphates if left in the tank. Waste water from the tank is brought down to the sump where it passes through the “socks” as they collect the heavier solids (excess fish food, waste, debris). The filtered water is then returned back to the tank via a pump. This system relies on the bacteria to be colonized in the live rock and substrate in the main display tank for biological filtration. These filtration systems are recommended for coral reef tanks as they give you a safe, and out of the view place, to keep unsightly equipment such as heaters, reactors, and even skimmers which we will explain next.

Protein Skimmer

A protein skimmer or foam fractionator is a device used to remove organic compounds such as food and waste particles from water. A protein skimmer uses foam refraction to remove protein based organics from the water before these organics can go through the nitrogen cycle and eventually end up as nitrates. This is where they are useful for coral reef keeping as nitrates much over 15 ppm can be detrimental.  The principal is based on injecting air into the water column  causing turbulence within the chamber of the skimmer. As the turbulence breaks the water into tiny air bubbles a foam is formed that allows nutrients and organic waste to “cling” to the bubbles to where as then carried up to a collection cup above for removal. This in turn helps with keeping the water not only clearer, but will aid in the nutrient load being lowered and provide for a near perfect environment for sensitive corals to thrive. These skimmers are also very beneficial to fish only systems as they remove the waste from the water column before they can break down and become an issue. 

Algae Scrubbers
While Algae Scrubbers are not a new idea, they are becoming quite the rage in must haves for the hobby. Basically, the concept of a scrubber is to trickle water across a screen while heavily blasting it with light to get algae to grow on the screen, in a controlled environment, thus absorbing nutrients such as Phosphate and Nitrate out of the water column as the algae grows. Once the screen is full, removing the built-up algae and discarding it creates the new growth cycle on the screen by absorption all over again. This concept creates a visually appealing display aquarium by growing controlled algae to out compete the nuisance algae that would otherwise grow in the display.

Overflow boxes are areas of the tank that are utilized for transporting the water from the tank, to the sump below the tank. Here again we have two types - overflow boxes and corner overflows.

The overflow box is essentially two boxes that hang on to the rim of the aquarium, using a tube or tubes under siphon to transport the water to the filtration area. These styles are typically used on fresh water tanks that are not drilled {and an additional piece of equipment that must be considered in the budget} when wanting to convert to a saltwater tank.

A corner overflow is a built in overflow where the tank is drilled at the bottom to allow the plumbing to come through the tank in a dedicated hidden chamber. This allows the water to be transported to the filtration in the most efficient manor - by gravity.