A Primer On Reef Tank Filtration And UV Sterilization

Choosing the right filtration for your reef tank is easy once you clearly identify your needs.

Choosing the right filtration for your reef tank is not all that difficult once you clearly identify your needs and study the various filter options available. Via daryl_mitchell /Flickr

It is said filtration is half of aquarium success. This may be true, but only if you select the proper filtration system for your particular reef tank. Selecting the correct filtration system can seem like a daunting task but it really isn’t that difficult if you examine and understand the basic types of filtration: biological, mechanical, chemical and ultraviolet (UV). A good filtration system maintains clean and clear water but more importantly, it maintains low levels of ammonia and nitrates, which have a negative impact on water quality. A properly filtered tank also includes excellent water circulation and surface agitation. Circulation and agitation provide vital gas exchange and whisks away damaging sediment.

Biological filtration is the most important type of reef tank filtration. It is the complex process of converting waste products, which are harmful to our reef tank inhabitants, into a less-toxic state. The biological filtration process consists of the nitrification and denitrification process. All of the surfaces in the tank along with the live sand and live rock in our reef tanks provide an ideal home for beneficial bacteria to conduct the nitrification and denitrification process.

Protein skimmer in sump
A protein skimmer in a sump. Photo by Stephen G. Noble

In addition to live sand and live rock, many reef tank keepers include a sump. A sump is simply a container of water below your tank. There are many possibilities for sump configuration but the most common are Berlin Method, wet/dry (trickle) filter or refugium. It is how it is used that specifically identifies its type. Regardless of the type, sumps add additional water volume to your reef tank and help to dilute pollutants. They also make it possible to house unsightly monitoring probes, heating equipment and pumps outside of your display tank creating a more realistic appearance.

The Berlin Method is the most conventional and simplest sump. Overflow water from the reef tank first passes through a micron mechanical filter (called a sock) which captures debris and algae and then empties into the sump’s main chamber. This style of sump can incorporate an internal refugium containing live sand, live rock, macro algae and even red mangrove trees. Water then flows into a protein skimmer (also called a foam fractionator) which removes organic waste before it can decompose into harmful ammonia, nitrate and phosphate. While we call them protein skimmers, this is somewhat of a misnomer. Protein skimmers remove more than proteins and the water is not skimmed on the surface. Protein skimmers provide chemical filtration through a process called adsorption. Waste materials called dissolved organic carbon (DOC) adhere to the surface of air bubbles and then deposit into a collection cup. Following foam fractionating, the water is then returned to the main tank via a water pump resting on the sump’s floor.

Wet/Dry Sumps

Wet/Dry sumps provide enormous amounts of biological filtration by having flowing water trickle over non-submerged media such as bio-balls which affords maximum air contact (aerobic) and biological activity. Over time, the bio-balls can become inundated with trapped debris producing enormous amounts of nitrates. It is for this reason, most experts recommend Wet/Dry sumps are best suited for fish-only marine tanks.


Refugiums can be used as a component of your Berlin sump, wet/dry sump or as a completely separate system. The refugium is teeming with life. Refugiums provide a refuge for macroalgae, copepods and amphipods to thrive in the absence of predators. Macroalgae such as Chaetomorpha are especially useful because of its ability to remove nitrates from the water. When it grows to a sizable mass, it is easily removed from the refugium and discarded. Many refugiums are lined with Live rock over a mud or deep sand bed. Some hobbyists choose to plant red mangrove (Rhizophora mangle) trees in their refugiums. Red mangroves consume nitrates and phosphates.

 UV Sterilizers

Ultraviolet sterilizers are a type of supplemental filter that contain an ultraviolet bulb sealed within a water tight quartz chamber. As water flows through the chamber, it becomes exposed to UV light which is deadly to free-floating parasites, algae and spores. UV assists in reducing the impact of parasite outbreaks but will not prevent the outbreak.

Choosing the right filtration for your reef tank is not all that difficult once you clearly identify your needs and study the various filter options available. I suggest new hobbyists consider not guessing but consult a trusted retailer, experienced hobbyist or aquarium club for assistance. This is especially important with the subjective aspects of filtration such as proper water circulation and skimmer size for your particular tank. Now that we have clarified one half of aquarium success, the other half being everything else, have fun and create a show piece for your home. Enjoy your fish!

Article Categories:
Fish · Reef Tanks