How Does A Wet Dry Filter Work? | Aquascape Addiction

How Does A Wet Dry Filter Work?

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We answer the common aquarium question of 'how does a wet dry filter work?', what it does and some important things that you should know about them.

How Does A Wet Dry Filter Work?

Wet dry filters are definitely interesting filters to go with for any aquarium. These filters are also called trickle filters because the water trickles down over the filter media. Wet dry filters are especially good for biological filtration.

While they can perform both chemical and mechanical filtration, it is not where they shine. These filters work best for biological filtration. So, let’s get to it an answer the question of how does a wet dry filter work?

How Does A Wet Dry Filter Work?

We do not want to go too deep into detail here, but enough to let you know exactly what is going on with these wet dry AKA trickle filters. We are going to keep things nice and simple and go through this in a step by step manner.

Trickle filters work using an overflow. This is some kind of mechanism which allows water to freely overflow from the tank into the wet dry filter once the water reaches a certain level. Once the water flows out of the fish tank it is collected by a drain pipe, also called a channel. This drain pipe then sends the water to the top of the wet dry filtration tower.

After the water enters the top of the tower, it enters some kind of horizontal spray bar or other horizontal distribution method. The water will then trickle down, which is why these filters are also called trickle filters. Most are either stationary spray bars or spinning spray bars. This mechanism can also take the form of a standard distribution plate. The job of the spray bar, spinner, or distribution plate is to evenly distribute the water across the top of the tower.

The water then proceeds to trickle over a variety of filter media (we have covered a detailed guide on filter media that you can find on this article). Now, we did mention how these wet dry filters are best for biological filtration. This is because the tower contains a fairly large open space with bio-balls that are suspended above the water. These bio balls engage in biological filtration by making the water pass beneficial bacteria. These bacteria clean the water as the water passes the bio balls, while also infusing your tank water with beneficial bacteria, the point of which is to kill ammonia, nitrites, and nitrates.

Yes, there are some wet dry filters out there that allow for mechanical and/or chemical filtration, but that is not their main purpose. Some wet dry filters come with mechanical filtration, usually in the form of a sponge and is generally located above the bio balls. However, when it comes to chemical filtration, few wet dry filters have it, with only the most expensive ones having the option to insert chemical filtration media.

After the water flows through all of the media, it flows into a sump (we have reviewed our top 9 Sumps over at this post), which is a fancy word for a collection tank. From there the clean water is then pumped back into the aquarium.

Why Is It Called A Wet Dry Filter?

Something to understand here is that wet dry filters are actually a specific type of trickle filters. Trickle filters are aptly named because the water trickles down over the bio balls. Now, wet dry filters are trickle filters that also have some kind of mechanical or chemical filtration.

They are called wet dry because the bio balls are suspended in the air, or in other words they are dry, while the mechanical or chemical filtration media is usually submerged in the water, or in other words, are wet.

Some Things To Know About Wet Dry Filters

There are a couple of things that you should know about wet dry trickle filters, so let’s just go over those things right now.

  • These filters are some of the best biological filtration units that you can get, but they are not ideal for good mechanical or biological filtration. Only the biggest and most expensive wet dry filters will allow for decent mechanical and/or biological filtration.

  • Wet dry filters generally require very little maintenance, but this is because they usually don’t have too much in terms of mechanical and biological filtration. At any rate, all you will need to do is cleaning the tubing on occasion and give the bio balls a good rinse.

  • It is a good idea to use a good mechanical and chemical filtration unit in conjunction with a wet dry filter.

  • Most wet dry filters allow for the addition of a heater, protein skimmer, and/or UV sterilizer (more on those here). This is especially true for the bigger models.

  • Wet dry filters are usually only ideal for larger aquariums of 60 gallons and up. They take up a lot of space and have a high flow rate, making them not too ideal for smaller tanks and slower swimming fish. However, there have recently been some wet dry filters released for smaller aquariums, but they tend to be fairly expensive.

  • You will need to add more water to your aquarium on a regular basis because wet dry filters tend to have a fairly high rate of water evaporation.


Wet dry filters are definitely very interesting and when it comes to biological filtration they work wonders no doubt. They have their advantages and disadvantages like other filter types too. Just make sure to do adequate research before buying one of these things as they are generally quite expensive to buy and set up.