A filter is something that helps clean the water. Contrary to how it sounds, for salt water tanks the main role of the filter isn't to catch particles from the tank to be removed. It is instead to house the bacteria which detoxify biological waste ( biological filtration) and also to push water through compounds which selectively bind specific waste products (chemical filtration). Filters can range from a protein skimmer, which uses complex physics and chemistry to remove nutrient from water, to a simple piece of sponge with water passing through it which pulls out debris and houses bacteria which break down this debris. A filter actually can even be a patch of living algae which grows by removing nutrient from the water. Many filters which you buy serve a number of functions ( perform different types of filtration and act as storage space to allow for filtering versatility) and we will go over the types of filters common in home aquariums before describing the mechanisms of what they are actually doing.



For your marine aquarium, depending on what you want to do, you still probably will not want to go to Walmart or Petco for a simple hang on tank or canister filter. These employ a number of the following filtration methods but are often too small, simple, loud, or crappy for the beginner trying to set up a sustainable healthy marine tank. Sample aquarium setups can give you a starting place for knowing what you may need for filtration and at what cost.


Sponge filters: Simple sponge filters are powered by the bubbles from an air stone creating lift and pulling a water current through a sponge. They are specifically mechanical filters which sift out detritus and provide a place for biological filtration to break that down. Very simple, these stay in the tank and are easily cleaned. These little guys are great for a bare-bottomed quarantine tank and overall cost about 5-10 dollars but you need an airpump to power them.


Hang-on-tank filters (H.O.T.): There are many companies that make these, Aquaclear and tetra: whisper being the most common ones you see. The best ones are the Marineland penguin/emperor bio-wheel hang on tank filters as they provide a high surface area well oxygenated environment on water wheels for bacteria to perform biological filtration. All of these filters have cartridges or inserts that the companies would like you to replace as often as possible ( range from 2-5 dollars each and can get expensive) however you don't need to replace these. When you do water changes, You can rinse the sponges/cartridges in the aquarium water you are about to discard. You can put whatever chemical filtration media you want into mesh bags or nylon stockings and load those into the “filter bay” area (where the cartridges normally go). With many of the cartridges (whisper and penguin), there is a clamp at the top which allows for you to empty charcoal out and replace in with whatever new chemical filtration media you'd like. The Aquaclear filters have the most space and easiest to customize filter bay area but realistically all of these filters are about the same. The most common mistake people make with these filters is not understanding that you need to prime them and that they take a minute to get going. When setting them up, you need to fill them with water so that the motor and impeller can pull air out of the inflow tube (video). The most common real problem in these and also in power-heads is a broken impeller. You will hear the motor spinning but no water movement is apparent. It is usually a problem with the magnet becoming detached from the propeller or propeller shaft. Here is a video on how to identify this problem and possibly fix it.

Canister filters: These are closed modules which pull water from your aquarium and push it through media in various filter bays before returning the water to your tank. They are all basically a canister or block with a pump inside and two hoses coming out, one to pull from the tank and one to push back to the tank. There are many places where a seal can fail and a siphon effect can empty your tank down to the level of where the water is drawn in by the filter (this is the case if the canister is below the tank and it's most common to hide these filters under the tank). It is for this reason that these filters are generally not a good idea, especially for long term and in a nice house. Eheim, Marineland, and tetra all have versions of these filters. These filters are also expensive and sold as “high end” however their propensity for disaster and difficulty for use, as well as their lack of large amount of space and customizability make them ill suited for any tank, especially a marine tank.

Wet dry filters: These are best for large aquariums especially fish only systems as they can accommodate larger amounts of wastes between servicing than the other filters. These units come in many different sizes but generally start at about 200 dollars. Ironically, you could make something just as functional with 20 dollars in Tupperware, egg crate, and silicone if you understand what is going on. The idea is that water from the tank trickles through a sponge, maybe a micron filter sleeve, then over high surface area bio balls or some other media like bio-pellets before filling up the bottom of the filter to be returned by a pump to your tank. This is the mechanical and biological filtration portion and these filters offer the largest space, surface area, and oxygenation for these to work. There is often space provided for heaters, skimmers, automatic top off switches, bags of chemical filtration media, etc in these filters. Many shapes and sizes exist but the basic principle is standard. Some folks take out all of the surface area and use the filters as just a sump or a refugium.


Sump: Similar in application to a wet dry filter, this is just a tank that is usually hidden where heaters, skimmers, etc can be working on the main system water. It also adds additional water volume to an aquarium system which increases stability of the water quality. A common reef sump is a 20 gallon long aquarium (for 65 gallon or less tanks) or a 40 gallon breeder aquarium ( for 75 gallon or more tanks) tucked inside of the stand where a refugium/algal scrubber can be hidden as well.


* Wet dry filters and sumps are basically the same thing. The main idea of the wet dry is high surface area sponges etc. to filter and process the water. A sump is just a place to increase water volume and hide stuff. Both can actually be done in the same receptacle (20 gallon tank or acrylic husk of a wet dry filter); the difference is what it's used for.


Protien skimming is a form of filtration and has it's own section.



The best advice for choosing a filter is to read the next section to really understand what filtration means, is, and does.



More about filters

There are three basic forms of filtration: Biological, mechanical, and chemical. There are a few other filtration methods such as the previously described protein skimming and the patch of algae (called an “algae scrubber”) but essentially they all aim for the same goal, make the water cleaner.


A natural coral reef is a well lit and highly oxygenated environment.  Nutrients are recycled without anything special for filtration because the filters are the animals and plants and bacteria of the ecosystem, all having evolved into a web which recycles nutrient and sustains itself.



Biological filtration is a method of using bacteria and other organisms (even your corals and worms count) to break down, process, detoxify, and re-incorporate wastes into a system. The nitrifying bacteria species, which we discuss in the starting a new tank section, are the fundamental component to biological filtration  because they basically detoxify fish urine. These bacteria rely on well oxygenated water and buffer. They also need a surface to do this on. Bio-balls, filter sponges, and live rock/sand are all surface area environments where nitrifying bacteria can live. This is a major component of most filters and overlaps with mechanical filtration in most cases (stuff gets stuck in a sponge then bacteria go to work)

A different type of biological filtration has become popular over the last decade. The practice of adding sugar/vodka to your aquarium also uses bacteria to process/re-incorporate different kinds of wastes (nitrates and phosphates) for removal from the system. The sugar or alcohol act  provides a carbon source to feed bacteria which at the same time allows them to incorporate nitrate and phosphate into their cell structure. Then protein skimming can physically remove those bacteria. Bio-pellets are a new type of product marketed by most of the big fish supply companies which also act as a slow release source of carbon (similar to sugar and vodka dosing) to fuel bacterial growth.



Chemical filtration is using reactive materials to remove toxins and other impurities from the water. Basically its putting little mesh baggies of rocks in your tank which absorb wastes. These materials act like magnets for specific or general junk that you don't want in the tank (though sometimes they also might indiscriminately remove some good stuff as well). Chemical filtration materials can be used in combination with other forms of filtration but are usually reserved in reef tanks for when there is a problem. Implementing them can be as simple though as putting the chemical/compound in a filter bag or nylon stocking and putting it in an area of high water flow (in front the return nozzle of a pump held in place by a clothes pin). Usually there is a place inside of the filtration area (sump, canister, hang-on-tank type filter, or even refugium) Activated carbon (charcoal) is the most popular form of chemical filtration and removes most biological molecules from water including hairsprays, hand lotions, bug sprays, dyes, and most organic molecules which come from animal waste in the aquarium. Most filter companies offer a variety of pre made cartridges for their filters however for marine aquariums people usually just hang a nylon filter bag or stocking in an area of high flow. There are many other chemical filter medias, some which target specific things such as phosphates ( phosban, phosguard, phosphate pad) or heavy metals (seachem’s copper removing resin). A whole category of water conditioners neutralize ammonia ( amquel, prime), which is very handy in an emergency.


Mechanical filtration is a mesh, pad, sand bed, or screen which traps particles before they are refrained from circulation and/or physically broken down to be removed from the filter. This usually goes hand in and with biological filtration unless the materials catching the particles is cleaned regularly as is the case with micros filters and some sand filters.


The refugium/ Use of algae for filtration:

A refugium is simply and area of the tank off limits to the normal inhabitant of the main tank. It is often used as a safe haven to let small shrimp or algae grow which can be slowly supplied to the main tank at the aquarists discretion. Using a refugium  to grow macro-algae (big visible types of algae not the single celled kinds which turn water green) helps with removing nitrates a phosphates which to them are like fertilizer. Chaetomorpha, a shiny spaghetti like algae, for algae the best in established systems, it grows quick and clean and you can remove it whenever but often herbivorous fish don’t eat it (emerald crabs, foxfaces, and some tangs and other inverts do however). It can actually fed out to these animals as a supplimentary source of food. Codium algae is also a good algae scrubber option as it grows slowly and can be manicured or removed easily. Caulerpa species of algae are often difficult to use as they tend to melt away and release nutrient randomly. This is usually avoided with 24 hour lighting, which is only a good idea in a sump (if constantly lit in main tank, fish would stress). Gracilaria and bubble algae pop up in abundance in some aquariums from live rock but are not generally used or counted as scrubber types of algae. pictures of algaes.

Protein Skimming.

 Skimmer filtration is the best form of filtration because it physically removes nutrient from water often before it is fully broken down and toxic. Skimmers do this without becoming exhausted and the only cost the initial equipment and a small amount of ongoing electricity. The various ratings for protein skimmers are arbitrary and, while some forms are more efficient than others and some even can pull more nutrient (they concentrate it better) than others, they all function by the same principal. A few good brands include eheim and octopus, though as just mentioned, as long as it's working a little bit, most skimmers all end up pulling the same amount eventually. Some advanced reef keepers only turn on their skimmers for a part of the day or even a part of the week or month. Skimmers pull out some good organic material which organism in our tanks would have been able to use and so allowing your  system some time for the the chance absorb and break down what can be used and tehn only occasionally skimming to remove the organic material which accumulates because nothing int eh tank can use it is a practical strategy. The time to do this is based on your system and often the skimmer is turned on when the most finicky coral looks a little bit unhappy (canary in a coal mine coral)


Mechanism of skimmer:

Water is polar and air is not polar in molecular arrangement. This means that stuff in water that has a neutral charge ( no + or - in the chemical equation) wants to be around air rather than water. This stuff sticks to air bubbles pretty well as it tries to escape water. What sticks to air bubbles even better are molecules that have a side that is polar and then another side (usually just a long carbon chain) which is non-polar. These line up in mats around bubbles with the non-polar side facing the air of the bubble and the polar side facing out towards water. Surfactants, like soaps and biles, are actually designed like this too and thats why they create froth when agitated. The longer bubbles bounce around in water, the more opportunity there is for these non-polar molecules to cover and coat the air bubbles.

Simply, a skimmer works by injecting a ton of bubbles into water thus allowing the slimy stuff in the water to coat the bubbles. As these bubbles eventually float up and accumulate at the surface, with some popping but adding their slimyness to the other bubbles around, they eventually overflow into a collecting cup ( the overflow stuff is called skimmate) and therby seperate the slime from the water. Water is let back into the tank somewhere underneath teh bubble matt (bubbles float water sinks).

Seriously, most skimmers will do. If youn have a 30 gallon or smaller fish or reef tank, the 100 dollar price range skimmers will be fine. Make sure that the powerhead which pushes water into the skimmer is a type which is equipt to draw in and pulverise airbubbles (called the venturi effect). Most powerheads actually can do this but they need a special inflow nosel  from where an airline can stem.

When you put oily food into an aquarium it often "deflates" all of the bubbles in teh skimmer stack. This is because pure oil bridges the bubbles to combine and essentially that all become one big bubble or 1 layer sitting on teh top of the water in teh skimmer. They stay there until bacterial can break down some of the oils or until the number of aliphatic molecules builds up again to create bubbles with integrity. I believe that once the large surfactant like molecules build up, they might also have an inner layer of pure oils which can eventually overflow into the skimmer cup as well.


Big picture conclusion:

The ideal filtration for reef aquariums is a natural, biological approach where reef critters and bacteria break leftover food down and then incorporate those nutrients into their own growth, just like nature. This paired with occasional protein skimming ( in our opinion an awesome form of filtration to remove those few things which no organism can utilize for growth) with the use of chemical filtration in cases of emergency is the ideal balance for a reef. Mechanical filtration happens with sponges or pads that water goes through if those are present in your filters however the junk which gets caught is processed biologically for the most part with only the occasional cleaning/wringing out of those sponges or pads to remove built up sediment which may cause blockage.

* It is best to rinse filter pads in tank water which you are about to get rid of ( water change waste water). If you use fresh water to rinse pads, all of the microbiology which inevitably remains in the pads stuck to the material will be dead and will decay in the tank over the next few days contributing to unwanted nutrient released into the water.



Sumps: Sumps are just an additional tank, usually out of site within the stand or in another room, which increases water volume and offers a place to hide filters/sponges/chemical dosers/auto top-offs. The thing I always worry about with water leaving the main tank is that there is another level of complexity to the system, more pumps and seals that can fail and cause a swamp in your living room. I like to keep things simple and when I do use sumps I try to keep everything within the sump with just one hose going in from the tank overflow and one hose going out back to the tank.


Now, after reading these sections,  you hopefully realize that there could be types of marine tanks which would use one of the simple brand name filters (such as a seahorse or very low density fish only tank with live rock) and some tanks which don’t even need any conventional filter (we run some reefs with just power heads and some quarantine tanks with just an air pump and air stone). The key is knowing what is happening biologically and chemically.