Wet/Dry Trickle Filters In Saltwater Aquariums

Aquarium fishes
Photography by Petya Todorova / Getty Images

Wet/dry trickle filters have been a popular choice in biological filtration for a very long time in the saltwater hobby. However, as the technology in aquarium keeping has advanced and the desire to attain a "natural" reef tank system becomes popular, controversy surrounds this choice.

Often referred to as nitrate factories, many aquarists believe that wet/dry trickle filters are suitable for fish-only tanks, but not reef systems. Over time the bio-material inside the wet/dry chamber becomes dirty, which eventually leads to a build up of unwanted nitrates in the aquarium, and as you should know, nitrates are not reef friendly! Therefore, to better understand this type of biological filter choice, let's take a closer look at how it works and its three basic components; the drip/trickle plate, the pre-filter (mechanical filter) set up, and the biomaterial used inside.

How Does a Wet/Dry Filter Work?

There are many designs to choose from when it comes to wet/dry filters, but they all work on the same concept. A wet/dry filter also referred to as a trickle filter or a bio-tower, is an anaerobic filtration method. For those of you that may not understand the term aerobic, it means occurring or living ONLY in the presence of oxygen. In other words, it can only work when oxygen is present. For this type of filtration, the more oxygen saturation it gets, the better it functions.

Water is pumped from the aquarium, then by means of a drip/trickle plate or rotating spray arm the water is dispensed or "trickled" down over and through a biological material source contained in the wet/dry filter chamber, but not before the water is pre-filtered by means of mechanical filtration, which can be accomplished with the aid of a protein skimmer or by placing a pre-filter material such as filter floss, a filter sponge, or micron filter felt on top of the drip/trickle plate area. When the water falls through the holes of the drip/trickle plate onto the bio-media, this allows for aggressive oxygen saturation of the water. Remember, aerobic! The clean filtered water is then deposited back into the aquarium either directly, or first into a sump or some type of water containment area and then returned.

The Importance of a Pre-Filter Set-Up

A pre-filter (mechanical filter) set up allows the tank water to be cleaned of excess debris, particulates and other unwanted organics (DOCs or dissolved organic compounds) before it passes through the bio-media in the wet/dry chamber. By using this process it helps to prevent the bio-material from getting clogged and dirty, as this is what can contribute to the build up of nitrates in an aquarium. A double drip/trickle pre-filter plate design, where one plate drips down onto the other, can also be incorporated. The first plate holds the pre-filter material on it, and the second is just a plain drip plate.

It is important to keep the pre-filter material changed. The purpose of the pre-filter is to only collect unwanted junk in the water, and nothing else. If the pre-filter material is not maintained regularly, this can decrease the flow rate of the water that trickles into the wet/dry filter, in turn decreasing the oxygen saturation level. This also allows the pre-filter material to act as a contributor to the build up of unwanted nitrates in the aquarium.

The frequency that you change the pre-filter material depends on the animal and feeding load you have on your system, but no less than a weekly change is recommended, and it definitely should be changed when it begins to turn brown.

Choosing a Bio-Material for a Wet/Dry Trickle Filter

Just about anything can be used as a biological filtration material, but ideally you want a type of media that has a large surface area for growing bacteria, does not compress down, is saltwater friendly, and has good water/oxygen exposure around and through it. The top choice bio-media is usually in the form small spiked plastic balls, or bio-balls, but there are many other types of bio-media one has to pick from. Some commercially sold wet/dry filters even come equipped with their own bio-media designed by the manufacturer to use specifically with their products. One example of this is CPR Aquatic, Inc.'s Wet/Dry Filters that come with CPR's trademark Bio-Bale.

If nitrates mysteriously begin rising in your aquarium, the accumulation of dissolved organic matter on the biomaterial contained in the wet/dry chamber may be the culprit. For this reason, it is important that you keep the bio-media free from DOC build up, which means it periodically should be cleaned. As discussed earlier, by having a good efficient pre-filter set up you should not have to clean the bio-media very often. 

Removing the Bio-Media

With the desire to attain a "natural" reef system, or that unwanted nitrate will become a concern, often aquarists will completely remove the bio-media from the wet/dry filter chamber AFTER their system has become well established and convert it into a sump box, allowing the live rock and sand to become the main biological filtration source. This can be done, but ONLY by removing about 1/4 of the media at any one time in at least 1-week intervals, testing for the appearance of ammonia in between to assure that the system is remaining stable during the elimination process of this biological filtration source. If ammonia does appear, DO NOT remove any additional media until the level drops back to zero, and then only continue on after waiting several weeks to allow the nitrifying bacteria population to build up again.

It is important NOT to add any new livestock to the system during this transition period as well. In our opinion a wet/dry trickle filter is an efficient biological filter choice, but only if it is properly maintained and other regular aquarium maintenance and water change routines are followed!