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Trickle Filters- Part 1

By Bob Goemans

Many different types of equipment have come our way in the past twenty years that have made marine aquarium keeping easier and more enjoyable, and in my opinion the trickle filter has had the greatest impact.  Even though there was isolated use of this type equipment in the 1970’s (deGraaf 1973; Moe 1973), it wasn’t until the mid 1980’s the benefits of the equipment caught the attention of the average marine aquarist.  When George Smit’s articles in 1986 appeared in Freshwater And Marine Aquariums (FAMA) they noted the benefits of trickle filtration and the “reef keeping" portion of the hobby was born and the quest for maintaining various types of corals went into high gear. Because we have learned that the combination of live rock and a protein skimmer is superior for popular reef system, trickle filters are no longer as popular as they use to be. However, they are still extremely useful on many marine systems, especially heavily stocked fish-only systems and commercial holding systems.

In Part I of this article I’ll discuss its general function, then how water gets from the aquarium to its trickle section.  Then how it is distributed at the top of the trickle section and the media it flows over.  In Part II, another form of trickle media is discussed along with its sump section and features one should look for when purchasing this type equipment. 

Trickle Filter Function

The trickle filter is usually composed of three separate units: the prefilter, trickle section, and sump.  Aquarium surface water first flows into a skimmer box or prefilter unit hanging on the side of the aquarium.  The mechanically filtered water then flows via gravity into the top portion of the trickle filter unit, usually located in a space under the aquarium.  When there, it enters some sort of distribution system, most often a stationary sprinkler pipe and is distributed over the surface of a drilled plate.  This “drip” plate is usually covered with some sort of mechanical filter material.

Water drips through the plate and enters the below trickle section where it flows over and between packing media that fills most of this section of the unit.  This section is usually referred to as the “dry” portion of the equipment because the packing media is not submerged.  The majority of the biological filtration and gas exchange in this type equipment occurs in this section.  The trickling of water in this so-called “dry” section provides oxygen to nitrifying bacteria living on the surface of its packing media.  This results in an effluent that is high in dissolved oxygen and nitrate.

When the water exits the bottom of the trickle/dry section it flows into a large open container referred to as the sump or “wet” section.  This portion of the equipment may be large enough to hold other filtering aids and a pump to return the water to the aquarium.

All in all, trickle filters allow for an unencumbered aquarium interior, free of various stand pipes, corner filters and air hoses.  In doing so it helps provide a more natural looking environment and increased dissolved oxygen which in turn helps promote a stress reducing and healthier environment for fishes and invertebrates. 


As for its individual sections, aquarium water first flows into the siphon/prefilter, sometimes called a skimmer box, where the water from the aquarium begins its journey to the equipment below the aquarium.  Even though this box can be thought of as a prefilter, its real purpose is to transport “surface” water from inside the aquarium to the top of the trickle section located below the aquarium.  These units are mainly designed as a siphon box that sit on the inside and outside top edge of the aquarium.  It also functions as a surface skimmer and can serve as a prefilter.

It benefits the aquarium in three ways.  It first removes surface scum allowing for a better air/water interface resulting in a better gas exchange at the waters' surface.  Second, it allows for upward flows in the aquarium that helps bring surfactant compounds towards the surface, improving the chances they will ultimately be removed by the trickle section or a protein skimmer if so equipped.  Thirdly, the upward flow of water is replaced by a downward flow of oxygen rich surface water to the animals near the aquarium bottom.

As aquarium surface water flows into the inside portion of the siphon box it is withdrawn from this area by a U-shaped siphon tube.  It is then transported over the edge of the aquarium and into its back section.  Water in this area then drains through a mechanical filter, usually a sponge filter, and flows via gravity downward through a connecting hose to the top inlet of the main trickle filter unit.

Some siphon boxes have the means to control aquarium water level and automatically remove any air trapped in its siphon.  Less expensive units simply keep pace with the volume of water being returned by the trickle filter’s sump pump.

Keep in-mind that the diameter of the siphon tube and that of the connecting hose must be large enough to keep pace with the volume of water returned to the aquarium from the sump pump.  If more water is returned to the aquarium then what can be siphoned out, the aquarium may overflow.  Of course, this depends upon the volume of water in the sump.  Siphons also have a way of collecting air at the top of their curved tube.  If the air bubble gets too large, the flow through the siphon tube will be reduced or stopped.

A short piece of airline tubing inserted through the inlet side of the siphon until its end reaches the air bubble can be used to suck air bubbles out.  Definitely not my favorite way of correcting this problem!  A much better way is by simply connecting the tube end to the small air inlet on top of a powerhead which in-turn will automatically suck out the bubble.  In fact, I’ve even seen some siphon tubes having small air release valves built into their curved section.  When opened, it allows the air bubble to escape.  The “C” Siphon from CPR automatically controls aquarium water level, muffles the sound of draining water, and removes any accumulating air in its siphon.  It’s my choice when it comes to an almost maintenance-free siphon box.

Keeping the mechanical filter media in siphon boxes clean is also important.  Dirty filter media can restrict water flow to the trickle section below and cause an overflow condition in the aquarium.  Some siphon units use a section of hollow sponge that surrounds its drain connection.  When the sponge becomes clogged, water simply overflows the open top edge of the sponge and flows into the drain line.

The sound of water entering the drain tube is sometimes quite noisy.  By placing a short standpipe drilled with many holes, water flow into the drain tube will become somewhat quieter.  All in all, prefilters are fairly simple devices and require little maintenance.

Unfortunately some trickle filters come with inadequate or no siphons/prefilters simply because the maker wants to save a few dollars so as to make their unit look like a better deal.  If there is no siphon/prefilter, beware!  Check out the cost of one and add it to the cost of the unit in question.  Then compare total cost to another brand that sells complete units.  Make sure a separately purchased unit will handle sump pump water flow.  If the trickle filter of your choice has a siphon/prefilter that looks kind of lacking in design, I suggest looking for another brand.

Aquariums can also be purchased in "reef ready" configurations or can be drilled and fitted with bulkhead fittings that allow them to drain surface skimmed water to a sump without the need for siphon overflows.  Many aquarists prefer these options for their "cleaner" look and reduced risk of flooding the tank.

Trickle Section

Way back when this type equipment first came to market, water from the aquarium would first flow into a single or combination of stationary PVC pipes that contained many holes drilled throughout their length.  Water flowed out these holes and ‘hopefully’ evenly covered the top drip plate of the trickle section.  Then came the tinkers who felt a better way was needed to distribute the water over the top of the drip plate and ‘invented’ the rotating sprinkler pipe.  But that not only added additional product cost, it led to much controversy.

Many aquarists reported that when plastic ball type packing was utilized some rotating sprinkler bars did not throw the water far enough to reach into the corners of the square-shaped trickle chamber.  Therefore the packing in corner areas never become wet and failed to become colonized with nitrifying bacteria, defeating a portion of the system.  In some cases they rotated too fast and threw the majority of the water to the sides of the container.  This allowed most of the water to run down the insides of the trickle section never wetting the central packing media.  And even if a roll of double layer spiral (DLS) material was used instead of bio-balls, some water dripped down in corner areas where it couldn’t come in contact with the roll of DLS.  Besides deficiencies in rotating mechanisms and ongoing debates about water distribution, the added product cost brought an end to rotating sprinkler pipes.  The less expensive way to distribute water won out, and besides, was more dependable and easier to maintain

Trickle Section Packing (Bio-Balls)

The most important factors as to trickle filter efficiency are the amount of surface area on the packing media below the drip plate and the even distribution of water when flowing over it.

The most popular form of trickle section media has been the plastic, usually ball-shaped items that got their start in air and waste treatment facilities.  Most of these media, properly called “packing,” were engineered to remove pollutants from smokestacks or sewage treatment plants.  Since this packing can provide surface area for the colonization of nitrifying bacteria, even those designed for smokestacks, they have progressed from their original function into the aquarium trade.  When used in aquarium trickle filters, the ever-moving thin film of water over the packing surface provides an excellent gas exchange area with the oxygen used by nitrifying bacteria living on their surface quickly replenished.  And, since these packing don’t wear out they are very cost efficient and need little or no maintenance.  Therefore, nitrifying bacteria living on this media receive all the oxygen they need and because of the trickling effect, highly oxygenated water flows back into the aquarium.

Of course this is not the case in the UGF system or the fluidized bed filter, where oxygen is removed by bacteria living on the gravel or sand particle and the water flows minus much of its oxygen back to the aquarium.

Whether this plastic material is shaped like a sphere, ring, cube, or whatever, “surface area” is probably the most important criteria for determining its cost effectiveness.  And surface area is generally measured in square feet on the total number of individual packing that will fit into a one-gallon container.  What constitutes how many gallons of packing are needed to support a given size aquarium has always been a good question.  You’ll probably find almost everyone has a different answer.

My rule-of-thumb is that a gallon of packing containing 10 sq. ft. of ‘useable” surface area is the minimum needed for every 10 gallons of water in a fish-only aquarium.  This is based upon my personal experience in my fish-only systems.  Depending upon the surface area provided by the brand media of your choice, the number of gallons required to fit a system must be adjusted.  For a heavily loaded fish-only system I would increase the recommendation to 1.2 gallons of packing per ten gallons of water.  As for their use in a reef aquarium the 10 sq. ft. of surface area should be applied to 20 gallons of water.  (But prefer such equipment not be used on reef aquariums because of the nitrate content produced.)  Usually, the packing manufacturer or distributor will or should supply that data.

Useable surface area is that which gets fully wet.  That depends upon its physical design.  This ‘wettability’ factor is extremely important, as nitrifying bacteria will not colonize a dry surface.  I don’t know of any chart or rating factors for "wettability", so I find the human eye to be the best judge.  I personally prefer the architecture of the ball-shaped packing and have used it in many of my past aquariums.

Another fairly important criteria is void space, which is the ratio of how much area is actually occupied by the packing structure and the space for water and airflow through the individual packing.  Void space is only important to the point of allowing for good distribution of water and airflow.  However, there are some that tout void space as an important ability of their product to degas ammonia.  After many discussions with environmental engineers on this subject, none believed this possible in the size equipment used on home aquaria.  These same engineers chose ball-shaped packing for their very large commercial stripping towers because of their excellent "wettability" and water and airflow characteristics.  In fact, all agreed that a structure eight feet high with a large volume of airflow would be required before even minute amounts of ammonia could be “degassed.”

And keep in mind that water and airflow must be evenly distributed throughout the packing that fills the trickle section.  Any type packing that would contribute to channeling, i.e., the concentration of either water or air in streams instead of even and sheet-like distribution, would distract from the media cost effectiveness even if it had greater surface area than other type packing.

There are some other points to remember when wanting to gain the most efficiency from the selected packing.  The first and most important is the application of an outside airflow through the packing.  By placing one or more airstones under the column of packing, dissolved oxygen in the aquarium can go from unacceptable to saturation.  Of course, when one does this the top of the trickle tower should have a vent hole for the additional air to vent.  And make sure all the packing is located above the water level in the sump.  Submerged packing, whether at the bottom of the trickle section or in the sump only detracts from the dissolved oxygen gained by the water evenly flowing over the exposed packing in the trickle section.

Trickle Section Packing (Double Layered Spiral)

Double layer spiral (DLS) is another form of trickle section packing.  It is a white polyester material sandwiched between layers of black woven plastic.  When new it has greater surface area for the same given space taken up by plastic ball-type packing.  It also allows for good airflow; does not restrict water flow; distributes water very evenly; allows for greater contact time; and, is less expensive than plastic ball-type media.  The key words here are “when new.”

Unfortunately DLS becomes coated with detritus and becomes a mechanical filter instead of a biological filter.  Prefiltering the water helps diminish buildup, but does not eliminate it on this type media.  And because of that, it periodically requires cleaning.  Yet even the most gentle cleaning methods will wash away much of its nitrifying bacteria since the material itself is very smooth and bacteria have no firm grip on its surface.  It will be weeks before it becomes recolonized.

The biggest problem with DLS is that it is used as a “roll” of media.  My personal opinion is the media should be sliced into pads, similar to slices of bread in a loaf.  Then, the entire trickle section could be filled with vertical “slices” of DLS, which would also eliminate the open corner situation in the trickle tower.  This would allow the removal of one or two slices per month for cleaning without radically affecting the overall efficiency of the entire filter as would happen if the entire roll were removed for cleaning.

There are some other considerations to take into account when using this inexpensive media.  First, the physical shape of the material makes it somewhat inefficient in high flow areas.  Because of water shear, i.e., the force generated by flowing water, nitrifying bacteria can be washed off its thin smooth string-like surfaces in high flow areas.  Therefore I recommend not using DLS in systems where there are high flow rates.  Also, since DLS appears not to degas carbon dioxide as effectively as do most plastic ball type media, I prefer to utilize this media in aquariums where macroalgae will be one of the more desired inhabitants.

Another factor to be considered is its initial cost.  DLS is very inexpensive when compared to most plastic ball type media, but will not last indefinitely (you know what I mean) as will plastic packing.  DLS will no doubt have to be replaced in a few years, increasing overall cost of the system and increasing time spent for general maintenance.

As to what constitutes how much of this type product would be needed to support a given size aquarium is somewhat guesswork on my part.  In my opinion, a layer of DLS that is .5 inch (1.5 cm) thick, 10 inches (15 cm) high and 25 feet (8 meters) long would provide biological filtration for an aquarium up to 100 gallons.  Another rule of “my” thumb is 15 cubic inches per gallon.

Keep in mind, just as it is preferable to maintain a constant upward airflow throughout a column of plastic ball type media, it is also preferable to supply a flow of fresh air through a column of DLS.  This can be accomplished by simply placing one or two airstones under the trickle section.  My dissolved oxygen tests have shown that a single plugged airstone can lead to a reduction of approximately 1.0 ppm of dissolved oxygen in the aquarium.  That is quite a large amount and could be the difference between success or failure in some systems.



After water flows downward through the trickle section it falls into the sump or what is usually referred to as the “wet” portion of the equipment.  Some aquarists are under the impression that by putting gravel or packing in the sump they can increase the biological capacity of the trickle filter.  True, but not a good idea when you think of all the trapped particulate matter breaking down into unwanted compounds.  If that is not reason enough not to place gravel or packing in the sump, consider the myriad of bacteria living on the surface of this material and the amount of oxygen they consume.  In the trickle section the oxygen is being replenished as water flows over and past the packing.  Not so with submerged media!  The additional oxygen the trickle section delivers to the aquarium is one of the main reasons this type equipment is purchased.  Why defeat it by having an unneeded large community of oxygen consuming bacteria living in the wet section. There are some important aspects to look for in a quality made wet section.  They are listed in the “Features” section below. 



As for some of the embellishments noted below, probably not all will be found on any single brand.  If they were, some might not be used or even needed and may only unnecessarily add to its cost.  Nevertheless, the aquarist could install those of interest if fairly handy with some simple tools.

* A large water reservoir/sump/wet section, because this type equipment encourages high evaporation.

* An overflow/prefilter so surface water will be constantly drawn off and filtered before entering the top of the trickle filter.

* Check to see if a sump water pump is included or an option/extra cost?  Remember that water flow through the trickle section should flow at about four times the volume of the aquarium.  Therefore a pump that has a flow rate of five to six times the volume of the aquarium will probably be needed.  This extra pumping capacity is needed because of flow reductions caused by how high the water needs to be raised above the pump (called “head”) so that it can be returned to the aquarium.  Curves and angles in the water supply tubing/piping also reduce flow as does wear on pump parts, as they become less effective with age.

* Is there an instruction manual.  Sounds simple enough but you would be surprised what questions can come up after you have the equipment at home.

* Look for a built-in connection for air pump tubing at the base of the trickle tower.  Some brands not only have the connection, but a distribution system, i.e., piping and airstones under the trickle section.

* Consider having a short length of vertical plastic pipe built into the drip plate to serve as an overflow.  Should the drip plate’s mechanical filter clog, water could buildup and overflow the top of the unit.  A short length of pipe will also serve as an airflow tube.  It allows the air that may be injected under the trickle packing to flow to the topside of the drip plate.  Note, you might want to place a vent hole in the top cover of the trickle section to allow the air to escape so its not forced to go up against the downward flow of water coming from the aquarium.

* Slide-out, draw type drip trays are a nice feature and make it easy to replace dirty mechanical filter media without shutting down and removing the top of the trickle section unit to perform this maintenance.

* Having a “Low and High” watermark on the sump side is also a nice feature.  These reference marks will let the aquarist know how close they are to having to add water to make up for evaporation.  It really helps to prevent salinity swings in the aquarium.

* Some units contain probe holders and places to attach float switches.  Nice if needed.

* Some also contain protein skimmers.  Look for a quality brand skimmer!

* Look for an angled deflection plate under the trickle section.  By directing the flow towards the opposite end of the sump any detritus that may slip through the trickle section will be deflected to an area in the sump where it is easier to remove.

* Find out if there is a real person at the place that manufacturers the unit and who will be available to answer questions.

* Does the unit have a manufacturer’s warranty?  If made by a reputable company there is no good reason why they should not stand behind their product.

* Is there a “weir” in the sump.  The weir is a solid plastic divider that basically separates the sump into two separate sections.  One section receives water from the trickle section.  Its water then overflows the divider/weir and flows into the section containing the return pump.  By dividing the sump into two sections should the prefilter clog, only the water that reaches the pump section will be returned to the aquarium.  The water on the other side of the weir will remain in the sump.  If the sump did not have a weir and the prefilter clogged, “all” the water in the sump would be returned to the aquarium and may cause an overflow.  Then try to explain the wet carpet to your spouse.


Whatever form this equipment takes, i.e., hang-on-back, a stand-alone unit, or built into the back wall or side of the aquarium, the trickle or wet/dry filter as many call it, is still a very useful piece of equipment. 

And if so equipped, it should be located in a dimly lit area so as to prevent algae from growing and clogging the trickle section.  Keep in mind algae growth on the packing material can cause channeling of the water and defeat the purpose of the unit which is to provide a means for nitrifying bacteria to carryout their function.

Nevertheless, because its final product is nitrate, it has fallen into disfavor with some reef aquarists.  I can understand that as keeping the nitrate level quite low is an important factor when maintaining the more complex reef aquariums.  And since protein skimmers, which almost all reef systems utilize, increase dissolved oxygen and reduce the nutrients that can be oxidized into nitrate, the need or usefulness of a trickle filter has become quite diminished on many reef systems.  However, where fish-only and general invertebrate systems are involved, and where nitrate levels are not of great importance, trickle filters remain an excellent type of biological filtration equipment.    

Wet-Dry, Trickle Filters  on WWM

Related Articles: Physical Filtration, Denitrification/Denitrifiers, Nitrates and Marine Systems

Related FAQs: Wet Dries 1, Wet-Dries 2, Wet Dries 3, Selection, Set-Up, Pumps, Plumbing Issues, Bio-Balls FAQs, Bio-Ball, Wet-Dry Media 2,Other Filter/Media/Elements (other than bio-balls), Modification/Conversion, Operation/Maintenance/Repair... Biological Filtration, Biofiltration 2, Fluidized Beds, Ammonia, Nitrites, Nitrates, PhosphatesDenitrification/Denitrifiers,


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