Saltwater Aquarium Filtration

Article by Shannon Goins

What kind of filter do I need for a saltwater aquarium?

There are two primary methods for filtering a saltwater system.

Method 1: Used for reef tanks, but also very effective with saltwater fish tanks

Live rock for microbial filtration (home for nitrifying bacteria) and a skimmer to remove excess protein. Often in this setup, aquarists will make use of a filter sock in the sump to perform macro-filtration and remove excess particulate matter from the water.

Some people utilize a refugium system to supplement their filtration. Refugiums take many forms, but essentially consist of a sump with a sand/mud base (such as Miracle Mud) with live rock, macro algae, and a light. Refugiums are very effective, but are not commonly used by the novice reef keeper. If you are interested in knowing more about how to set up a refugium, your local fish store’s expert staff can help you.

Method 2: Used primarily for fish only systems

Bio-chamber, or wet/dry trickle filter system, which utilizes bio-balls in the sump as the area for nitrifying bacteria to colonize. Wet/dry filters are often topped with a tray where filter floss can be used to perform macro-filtration. A skimmer is also needed in fish only systems to remove excess protein from the water.

How much live rock do I need?

A minimum of 1 pound of live rock per gallon of water is recommended to provide adequate filtration, but you may exceed this minimum recommendation. The more live rock, the better, providing that you still have adequate flow in your tank. Be sure that your powerheads move water in all areas of the tank if you choose to have a significant amount of live rock.

Depending on the density of the rock, 1 lb per gallon may or may not provide you with enough rock to make a nice aquascape. Either select at least some live rock that is very open in structure or consider adding some dry lace rock to round out your aquascape.

Lace rock comes from the ground and is usually purchased dry. There is also the option of purchasing dry versions of live rock. This rock has been completely dried and has no living matter on it. Some local fish stores will seed dry rock by placing it alongside live rock. “Seeding” is the process of exposing dry, lifeless rock to an environment rich in bacteria and other live rock critters that slowly make their way to the dry rock. After several weeks in an aquarium or live rock vat full of other live rock, dry rock becomes “live” and may be counted toward the minimum 1 lb per gallon rule in setting up one’s aquarium.

Using some dry rock also makes for a cheaper setup in a saltwater aquarium, because dry rock costs about 1/5th the price of live rock. This is one corner that can be cut in trying to make a saltwater tank more affordable in the setup phase. However, using dry rock requires more patience in their tank setup because lace rock cannot be counted as “live” for several weeks and does not contribute to reducing ammonia until it has been well colonized by nitrifying bacteria.

What is meant by “cured” and “uncured” live rock? How do I know the live rock I am buying is cured?

Live rock is kept moist, but not totally submerged during shipping and arrives to our store with many of the life forms still alive. Be aware however, that live rock must be cured before you can put it in an aquarium that already has fish and invertebrates in it.

Curing is the process of waiting for organisms on the live rock that did not survive shipping to die and fully decompose. This process involves very high levels of ammonia and is best done at your local fish store. It is possible to start an aquarium with uncured live rock; however, there is no real benefit by curing the rock yourself. Additionally, you must put up with the rotten egg smell it produces.

There are two things you should do to determine if your live rock is cured. First, ask the store when they got the live rock in. In a very well established system with excellent skimming ability, it takes about 1.5-3 weeks to cure a piece of rock. This also varies depending on the density of the rock, how much cured rock is in the vat with it, how much filtration is on the system, and how much flow is moving through the rock.

The second thing you should do to determine if the rock you want is cured is to smell it. Cured rock has a very musty, earth-like scent to it. If the rock smells like rotten eggs, put it back and come back in a few days.

Do not believe someone who tells you that they just got in cured rock. It takes a few days for it to smell bad and unethical vendors might try to push it out the door before they should. Likewise, “cured” rock purchased on the internet may be cured, but if it is not shipped TOTALLY submerged in water, you will have to cure it yourself. These vendors are very rare, mostly because it is cost-prohibitive to ship rock fully submerged due to the weight.

Can I use an undergravel filter with my saltwater aquarium?

Generally undergravel filters are no longer used in saltwater systems. While this was the practice 15+ years ago in saltwater, the hobby has since developed more effective filtration methods.

How important is a sump?

The sump is the center of all complete saltwater filter systems. Using a filter pad or a filter sock can trap waste from the water as it enters the sump from the overflow above. We recommend purchasing the largest sump that will fit in your aquarium stand. Having extra space in your sump makes it easier to maintain your aquarium. Filter systems using in-sump protein skimmers and/ or submersible pumps require larger sumps than systems with in-line (external) pumps and hang-on or in-line protein skimmers. If you have room, you may place pieces of live rock in the sump to act as additional biological filtration.

How does biological filtration work?

Biological filters work by growing bacteria, which break down fish wastes. Nitrosamonas and Nitrobacter break down ammonia, which is very toxic in alkaline water, to nitrites. Nitrites are then broken down to nitrates, which are much less toxic. Nitrates are removed by water changes.

What is a plenum? What does it do? Is it good or bad or indifferent for a saltwater aquarium?

A plenum is a form of biological substrate filtration that lies under the substrate that removes organic contaminants such as nitrate. The general consensus is that plenums are very sensitive and become unbalanced very easily. Their usefulness is often regarded as dubious.

Although plenums might help remove nitrates from an aquarium, the risk of having one greatly outweighs potential possible benefits. Nitrates may be removed from the water above, but the contents of the plenum are extremely noxious and dangerous to the aquarium’s inhabitants.

When plenums rupture, attempts to rescue fish and coral from tanks that have very suddenly taken a turn for the worse can often be in vain. Typically within 24 hours of the owners having suspected there was a problem, the aquarium is full of dead or dying corals and fish that have “nuked”. It is usually too late for most of the fish and coral.

The cause? A slightly disturbed plenum. Either a sand sifting critter or human has created a small hole in the plenum that has allowed sulfur dioxide to leach out in to the aquarium. In a single word, the result is devastating.

Can I use a canister filter on my saltwater aquarium?

Yes. You may choose to use a canister filter if you wish. There is no harm in doing this if you maintain it regularly. Canister filters help serve as a method of macro filtration, and can help to polish the water, making it clearer. In reef tanks, the fine pad may be omitted to allow very small desirable particulate matter to reach the live corals.

That being said, canister filters are not a required part of a saltwater tank. The most important form of filtration for a saltwater system should be provided either live rock or a wet-dry filter in the sump combined with a skimmer.

I am setting up a fish only saltwater system. I am confused about whether or not I should use live rock or a wet-dry system for biological filtration. Could you tell me the difference? Also, do I need a canister filter for a fish only system?

Wet-dry filters do an excellent job of breaking down organically produced ammonia and nitrite. They also drive off carbon dioxide and add oxygen to the aquarium.

Live rock may replace the function of the bio balls used in a wet-dry system. Live rock produces less nitrates, which is the only downside of a wet-dry or trickle system. Live rock, however, does not handle the same amount of organic waste that a wet-dry filter does. For this reason, live rock alone may not be suitable for a heavily stocked fish only aquarium, unless the aquarium has a very strong protein skimmer.

Canister filters do a better job of removing particulate matter than a wet-dry filter alone. For a fish only tank they remove fish waste and inorganic particulate matter. Changing the filter pads regularly disposes of these materials. In addition, canister filters provide supplemental biological filtration and a small degree of circulation. You may choose to add a canister filter if you wish.