Let's Talk Filtration (And How 'Filter-less' Aquariums Work)

[Updated 11/03/2025]

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Have you ever wondered how water is 'kept clean' in a filterless aquarium? Truth be told, the only case of truly filterless aquariums are essentially empty jars with water + live stock, as filtration still occurred in 'filterless aquariums,' just that it is not via a conventional 'filter.'

So let's start with defining what 'filtration' and, subsequently, what a 'filter' entails.

Defining filtration

To talk about something, we need to know what we are even wanting to talk about. Ultimately, so many things in this hobby are confusing since discussions often involve very different expectations. Take cycling for example and the myriad of definitions it has had over the years. Hence, the definition of 'filtration' here is so important.

So what is 'filtration'? Well, at the highest level, for me:

Filtration is the process in which unwanted substances are removed/sequestered from a system.

This may seem obvious, but it is very important to what we talk about here. First, different setups may see 'unwanted' vary. Tannins, for example, are not harmful, and some suggest they may even have health benefits. At the same time, it colors the water brown, and not all find that aesthetically pleasing. So, while some aquarists adore tannins and their coloration, others consider them an impurity to be filtered out. A different example is nitrate, which we do not want too high. However, it is also a source of nitrogen for plants, corals, etc.; therefore, in some setups, it is even purposefully added.

Every once in a while, you may see aquarists mention that certain types of shellfish are used as very effective 'filters' in some systems and ask if they can rely on shellfish for filtration only to be met with a hard 'no,' that's because the purposes/goals can be very different. Some systems only care about filtering specific types of heavy metals, and certain types of shellfish are very efficient at removing them. However, the same shellfish may not help filter out other unwanted substances, like ammonia.

That's why it's so important to define 'filtration' here and, subsequently, how you define it for your specific aquarium. For the sake of this article, filtration is defined as any mechanism to remove unwanted substances from your aquarium.

The types of filtration

Filtration can generally be classified as mechanical, biological, or chemical.

Mechanical filtration

Mechanical filtration is where unwanted substances are physically removed/sequestered from the water. Examples include filter socks, skimmers, filter floss, etc., which literally trap matter for subsequent removal. These can be great at keeping the water clearer by removing particulate matter. However, they may not help with chemicals such as ammonia, nitrite, nitrate, phosphate, etc.

Biological filtration

Biological filtration is defined as the reliance of organisms to remove/sequester unwanted substances. The most commonly discussed types of organisms involved in biological filtration are nitrifiers, responsible for the oxidation of ammonia to nitrite and then subsequently to nitrate (sometimes in a single step). However, they are not the only organisms that can be beneficial in the aquaria. Biological filtration can involve, but is not limited to:

• Nitrification (of ammonia to nitrite and nitrite to nitrate).

• Denitrification (of nitrate potentially all the way to nitrogen gas).

• Energy metabolism of organic substrates.

• Assimilation of carbon, phosphorous, nitrogen, and other chemicals for growth.

• Removal of pathogenic or otherwise nuisance organisms.

Many microorganisms can break down organic substances in the aquarium to prevent the build-up of harmful chemicals, which are often packaged and sold in products such as Microbe-Lift Special Blend. It should be noted, however, that there has been no robust evidence that using such products is necessary. From my experience, the moment you stock the tank, that's enough to introduce such microorganisms into the aquarium if they were not already there.

Nitrifiers are the only beneficial microorganisms I would purposefully add to an aquarium. However, not all products claiming to help with the cycling process are good. Here are the ones I would recommend:

• FritzZyme TurboStart 700 is the best for freshwater setups. FritzZyme 7, the less concentrated version, works, too, but not as well.

• Tetra SafeStart+ also works for freshwater. It is not as good as FritzZyme TurboStart 700, but it works.

• For saltwater, FritzZyme TurboStart 900 and Bio-Spira work a treat. FritzZyme 9 works, too, but not as well as FritzZyme TurboStart 900. Side quest: if you are interested, a nifty series of experiments is done by an aquarist who goes by the username Taricha on Reef2Reef here that investigates the efficacy of various potential nitrifier sources in terms of cycling. It is a good read if nothing else.

One important thing to consider is that microorganisms, especially nitrifiers, need a place to live. That's where biological filter media, or just 'biomedia,' comes in. Biomedia is designed specifically to house these microorganisms.

It should be noted that any surface in the aquarium can harbor beneficial microorganisms. A study by CerMedia suggests one can obtain more than enough nitrification with literally an empty aquarium. Some food for thought! But when we talk 'biomedia,' we often refer to products specifically designed to harbor beneficial microorganisms and do that well. This is why, although products like filter floss can offer both mechanical and biological filtration concurrently, most aquarists rely on it for one or the other. This is because filter floss designated for mechanical filtration can often be removed when clogged up, and certain types of filter floss need to be removed as they are not designed for sustained use and will break down over a relatively short time.

Ironically, filter cartridges break down relatively quickly despite being marketed for biological filtration. This is why you may have seen, rightfully, recommendations to swap them out for other biomedia as soon as possible (preferably when setting up the tank), as obviously, filter cartridges are actually not suitable biomedia.

There are products, in fact, designed for prolonged use as biomedia. The best one I have personally tested is CerMedia MarinePure by a long shot. These offer significant surface area, and therefore, you can, in fact, get away with a small amount to filter large water volumes. There is evidence to suggest they leach aluminium, but at the same time there has been no evidence to suggest the amount leached, if any, causes harm in any aquarium setup. Another common product is Seachem Matrix. Note that both CerMedia and Seachem touts that their products can offer denitrification alongside nitrification, however so far there has only been limited evidence to suggest the MarinePure blocks can offer denitrification. Due to the limited evidence, I urge anyone buying these or any other biomedia products only to buy them with the express expectation of nitrification, not denitrification.

On that note, yes, shellfish and other organisms that consume organic matter (for example, bits and pieces of food) in the water are considered to engage in biological filtration. Many organisms, such as plants, algae, and corals, also consume ammonia and nitrate and, therefore, engage in biological filtration. Yes, I did mention algae, and yes, some people indeed rely on algae for filtration, often in the form of an algae scrubber, but also in refugiums (the latter also containing other beneficial organisms).

Detritus worms, snails, or similar are not considered to engage in biological filtration, as they produce waste from their metabolic processes. In a sense, they are swapping one thing unwanted for another, although certainly, they can be helpful in the eventual filtration of unwanted matter.

It should be noted that the 'mulm' that accumulates on biomedia and other surfaces, unlike conventional knowledge, does not seem to contain a lot of nitrifiers. Plenty of aquariums have spotless biomedia, and yet plenty of nitrification occurs. Vice versa, plenty of aquarists regularly rinse all of the mulm off of their filter media to zero impact on their nitrification rate. Nitrifiers are very efficient, and their colonies are unlikely to grow to a size visible to the human eye. So yes, likely all that 'mulm' is just detritus + microorganisms that when removed, most likely have a neutral or beneficial impact.

Chemical filtration

Sometimes, mechanical or biological filtration does not remove some substances, such as copper. Chemical media reacts to such substances, either converting them into a harmless chemical or something that can be sequestered in the media. Due to the requirement that a reaction has to occur, chemical filter media are used up, which is why they may need to be periodically replaced.

UV sterilization

A fourth type of filtration, which is unique in and of itself, is the UV sterilizer. Generally, they work by pushing water through them, exposing organisms to UV and killing them. Note that certain purple-colored lighting is sometimes called 'UV' light, but they are technically not. This can cause confusion, and aquarists have worried that their 'UV' light over their tank hurts their livestock or themselves, when in reality, such lights are just a deep purple. In contrast, some aquarists have put actual, true UV lighting above their tank, and... the results were not great. DO NOT EXPOSE YOURSELF TO REAL UV LIGHT.

Armed with this knowledge of the variety of filtration mechanisms, you should now have a better idea of the different products and how to differentiate between them. Poly-Filter, for example, looks like some types of mechanical or biological filter media and can technically work as either, but is rather chemical filter media. So it would not be surprising that aquarists replace this product regularly.

What filtration is required?

The above description of filtration types may suggest that all three types of filtration are required for all aquariums. However, this is not necessarily the case.

Many, if not most, aquariums do not really need chemical filter media. Chemical filter media can remove certain substances like nitrate or phosphate. Still, many of these substances can be removed by biological filtration, often without having to replace the media itself regularly, hence why many opt for biological filtration as the preferred method for removal of these substances instead. Chemical filtration is often used to remove medication dosed directly in the aquarium once the treatment is done - though large water changes generally also suffice.

Many aquariums also do away with mechanical filtration or at least only use it occasionally, either because it does not produce much floating matter or because it does not impact how they care for or enjoy the aquarium.

The only real filtration that is required would be biological.

So, what is a 'filter?'

Based on how we define 'filtration,' a filter would be anything that performs filtration, i.e., filtering the water of unwanted substances. Note that I specified 'anything' rather than 'contraption,' 'mechanism,' etc. This is very important.

Aquarists conventionally imagine filters as objects where water runs through them and said objects filter the water. This applies in many cases - internal filters, hang-on-back filters, canister filters, overhead filters, under-gravel filters, sump filters, and to an extent even sponge filters (though the last of these do not rely on water pumps, but air pumps, to function). There are, in fact, even more types of 'conventional' filters than that, but I am just listing a few to give an idea of what we normally imagine.

Let's discuss a specific marine setup, a 'fish-only with live rock' (fowlr) aquarium. Many of them run off of nothing but a water pump/circulator and live rock (don't worry about what live rock is if you are not familiar with the term; they are just super porous rock). So then... what is the 'filter' in this case, the water circulator? Well not really, no (intended) filtration occurs when the water flows through the water circulator. It is actually the live rock that is doing the filtration, specifically biological filtration. So then, would the live rock be the 'filter' in this case? Well, it can't be. Without the water circulator, the live rock would probably not perform nearly as much filtration as one would aim. In this case, the entire aquarium is often considered the 'filter' in and of itself.

This is the first step in re-visualizing what filtration in an aquarium setting means and why it can be important to consider alternatives to the 'conventional' filter when considering filtration. It is also why, when thinking about filtration, you should think about the filter media and so on themselves rather than the actual contraptions holding the media.

For example, consider whether a canister filter is truly necessary if the biomedia you expect you need fits in an internal filter.

Let's take this photo of a section of one of my aquariums. What you see in the picture is essentially my filter. You can probably make out the pump. That's not the whole filter, though. All it does is pump water up to the top of the piece of driftwood, where I have cotton pads covering the outlet. The cotton pads here act as mechanical filtration, so you can see they are entirely brown. I replace them often. So what about the biological filtration? I did mention they are really important, right? If you guessed the moss to the left of the picture, you guessed right. The water runs down the driftwood and essentially fertilizes the moss. The moss is the majority of what consumes the nitrogenous compounds in my tank!

Yep, that's my filter. If you think about it, it still follows the convention of a typical 'filter' you may imagine. Yes, there is water circulation. Yes, there is mechanical filtration. Yes, there is biomedia (the driftwood!), and yes, there is biological filtration.

Filter-less aquariums

So, let's talk about 'filter-less aquariums'. These are aquariums with nothing you'd traditionally recognize as a 'filter', sometimes not even a bubbler or a water circulator. But are they truly 'filter-less'?

At this point, you probably had the answer yourself - no, they are not truly 'filter-less'. The whole aquarium is a filter. Plants, algae, and other organisms perform all the biological filtration needed. Many are lightly stocked but can also be more heavily stocked, relying on more plants and other organisms to ensure that the water is still safe for livestock in the aquarium despite no water circulation and no other types of filtration.

That's it! No magic, nothing crazy here. Just realizing that the whole aquarium is a filter.