[Note: This article is in draft-ish form. I would very much appreciate recommendations for how to improve this article. Feel free to comment or message!]
For anyone looking to set up an aquarium, it is almost a given they would hear a filter is a necessity - along with a heater, water conditioner, and the aquarium itself. Then eventually, one would hear of 'filter-less' aquariums too. So what gives? What's the deal? This article aims to explain filters - what they are, what they comprise, and finally - how 'filter-less' aquariums work. Which turns out, they are not quite so 'filter-less'.
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 why 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 like an obvious description, but is very important to what we talk about here. First, different setups may see 'unwanted' vary. Tannins for example, may be unwanted to some, but desirable by others - because tannins while not harmful, may be unappealing and to some, having that brown discoloration go away is important. A different example is nitrate - which may be undesirable in purely fish-only aquariums, but are desirable in planted or reef tanks, whereby nitrates can be an important nitrogen source for growth of photosynthetic organisms and those that rely on them for survival.
Every once in a while you may see aquarists mention that certain types of shellfish are used as very effective 'filters' in certain systems, and ask if they can just 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 out certain types of heavy metals, and certain types of shellfish are very efficient at removing them. However, the same shellfish may not help with filtering out other unwanted substances, like ammonia.
That's why defining 'filtration' here, and subsequently, how you define 'filtration' for your specific aquarium is so important. But yes, ultimately filtration is just any mechanism to remove unwanted substances from your aquarium.
Defining a 'filter'
Based on how we define 'filtration', then 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.
How aquarists conventionally imagine filters, would be an actual object where water runs through it and said object filters the water. This applies in many cases - internal filters, hang-on-back filters, canister filters, overhead filters, undergravel 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.
But let's talk about 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 are 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 be performing nearly as much filtration as one would be aiming for. 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 is can be important to consider beyond the 'conventional' filter when thinking about filtration.
The various types of filtration
Filtration can largely be broken down into three categories: mechanical, biological, and chemical.
Mechanical filtration is where unwanted substances are physically removed/sequestered from the water. Examples include filter socks, skimmers, filter floss, etc., things that literally trap matter in them for subsequent removal. These can be great at keeping the water clearer such as of particulate matter, but do not directly remove substances such as ammonia, nitrite, nitrate, phosphate, etc.
It should be noted that certain types of filter / filter media can engage in multiple types of filtration at once.
Biological filtration is defined as the reliance of organisms to remove/sequester unwanted substances. The most commonly talked about 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 engage in break down of organic substances in the aquarium to prevent 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 serious evidence that using such products are necessary. They can certainly be beneficial, but it seemed like just introducing fish or other live stock to an aquarium introduces enough beneficial 'waste-consuming' microorganisms that reproduce quickly enough that it isn't really important. Introducing nitrifiers via a bottled product can definitely work, and is recommended. However, not all products are good. Here are ones I would recommend:
FritzZyme TurboStart 700, for fresh water - the best of these line of products on the market right now. FritzZyme 7, the less concentrated version, works too, but not as well.
Tetra SafeStart+ works for fresh water too. Not as good as FritzZyme TurboStart 700, but works great.
For saltwater, both FritzZyme TurboStart 900 and Bio-Spira works a treat. FritzZyme 9 works as well, 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. A good read, if nothing else.
One important thing to consider, is that microorganisms need a place to live, especially nitrifiers. And that's where biological filter media, or just 'biomedia' comes in. These are products that are designed to house nitrifiers (and other beneficial microorganisms). It should be noted that really, any surface in the aquarium can harbor beneficial microorganisms, and indeed 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 do often refer to products specifically designed to harbor the beneficial microorganism, and do that well. This is why although products like filter floss can offer both mechanical and biological filtration concurrently, most aquarists often rely on it for one or the other. This is because filter floss designated for mechanical filtration can be often removed when they are clogged up, and certain types of filter floss in fact needs to be removed as they are not designed for sustained use and will break down over a relatively short period of time. This also include filter cartridges in many filter products, which seem to be designed to have an especially short lifespan. So even when considering products specifically described as biomedia, always do research to determine if you may want to use them. 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 would urge that anyone buying these or any other biomedia products only buy them with the express expectation of having a place for nitrifiers to live.
On that note, yes, shellfish and other organisms that consume particular matter in the water are considered to engage in biological filtration; as do plants, algae, and other photosynthetic organisms consuming nitrate and other unwanted chemicals in the water; but technically not detritus worms, snails, or similar when talking about their consumption of larger, non-floating matter. 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).
It should be noted that the 'mulm' that accumulates on biomedia and other surfaces, unlike conventional knowledge, do not seem to comprise much nitrifiers. Plenty of aquariums have biomedia that are spotless, and yet still plenty of nitrification occurs. Vice versa, plenty of aquarists regularly rinse off all of the mulm off of their filter media to zero impact on their nitrification rate. Nitrifiers are very efficient, and it is unlikely their colonies really grow to a size visible to the human eye. So yes, likely all that 'mulm' is just detritus + microorganisms that even if removed, negligibly impact the health of your aquarium.
The last type of filtration is chemical, whereby the media used reacts with the unwanted substances in some way, either turning the unwanted substance into something no longer a problem and/or sequester the resultant product within the media itself. Due to the requirement that a reaction has to occur, chemical filter media are used up, and therefore the reason why they may need to be periodically replaced.
A kind of fourth type of filtration, and kind of unique in and of itself, is the UV sterilizer. Most work by pushing water through a contraption with an UV light that kills living organisms exposed. Do note that sometimes certain purple colored lighting are also called 'UV' light, but they are technically not. This can cause confusions, and there has been aquarists worried that their 'UV' light over their tank hurts their live stock or themselves, when reality such lights are just a deep purple. In contrast, some aquarists have put actual, true UV lighting above their tank and... results were not great.
Armed with this knowledge of the variety of mechanisms of filtration, it is then important to understand the purpose of different products, and how to work with 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 are 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 be used to remove certain substances like nitrate or phosphate, but many of these substances can be removed by biological filtration, often without having to regularly replace the media itself, hence why many opt for biological filtration as the preferred method for removal of these substances instead. Chemical filtration does have an use, often to remove medication dosed directly in the aquarium.
Many aquariums also do away with mechanical filtration, or at least only use them every once in a while, either because they don't have much floating matter produced, or because it does not impact how they care for or enjoy the aquarium.
The only real filtration that is required would be biological.
Fish-less aquariums
So let's talk '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 a higher number of plants and other organisms to ensure that despite no water circulation, no other types of filtration, the water is still safe for live stock in the aquarium.
That's it! No magic, nothing crazy here. Just realizing that the whole aquarium is a filter.