Do I need a filter for an aquarium?

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The aquarium filter is considered one of the most essential pieces of fishkeeping equipment. But are they necessary in all cases, and how do they work?

Words by Nathan Hill

Aquarium filters can be essential for both freshwater and marine tanks. They help to remove physical and chemical waste products like fish excrement, uneaten food or plant debris which contain toxins such as ammonia and nitrate. This waste can collect in the tanks, contaminating the water, and becoming potentially fatal to the fish in your tank. And so, filtration is a common method used for maintenance of healthy, happy tank. 

Why do I need an aquarium filter?


When we think of an aquarium filter, we tend to imagine a physical device of some sort—an air- or motor-driven canister filter inside or outside of the tank, a sump underneath it, or (for the old school aquarists) even a plastic plate that sits underneath the aquarium substrate.

The best way to think of filtration is in terms of nutrient import and export, and in-tank nutrient management. Whenever we add something to our aquaria, such as food, we are adding nutrients that will gradually accumulate and become toxic. 

While the purpose of tank maintenance (especially water changes) is to export excess nutrients back out of the tank, the purpose of an aquarium filter is to remove or alter waste products from fish (the natural outcome of our giving them nutrients such as food), rendering them harmless to the tank’s inhabitants until they can be exported.

There are three ways in which this rendering harmless can occur: biological, mechanical, and chemical filtration.

Related article: 39 Top Tips for Fishkeepers

Biological filtration

This is what most people in aquatics consider the default filtration technique. Biological filtration utilises the natural function of several different nitrifying bacteria (and possibly archaea too) to convert dissolved, toxic fish wastes into less toxic wastes. 
 
Specifically, biological filters deal with the conversion of ammonia (the toxic waste excreted by fishes, as well as created by the decomposition of decaying organic matter like dead plants or fish food) into nitrite, and subsequently into nitrate. 
 
While all three of these are toxic to fish, their level of toxicity varies. For ammonia, as little as 0.02mg/l of free ammonia can be dangerous to fish. For nitrite, anything above 0.2mg/l can be dangerous. For nitrate, nitrate toxicity appears to be species dependent, though it’s generally accepted that levels should be below 50mg/l for most aquarium fish. 
 
The bacteria that convert ammonia through to nitrate need a substrate to live on, and this is usually known as biological filter media. Biological filter media is designed to have a high surface area to volume ratio, and it often porous—the more surface area that’s available, the more bacteria that can grow. An inadequate amount of media (or surface area) will result in insufficient bacterial growth, and ammonia and/or nitrate accumulating in the water. 

Mechanical filtration

A mechanical filter is any filter that physically removes solid, particulate waste from the tank. These solid wastes can then be removed before they can decompose, reducing the amounts of dissolved wastes like ammonia and reducing the burden on biological filters. 
 
Mechanical filter media is usually in the form of foam or wool pads or blocks. It’s usually located in the aquarium filter before the biological filter media, so as to stop the biological media from becoming covered or clogged (and subsequently reducing the amount of available surface area). 
 
Mechanical filter media needs to be cleaned regularly—as it collects waste, it will reduce the outflow of pumps.

Some filters rely on a mechanical filter media such as a sponge to function in both a mechanical and a biological role. In these instances, extreme care has to be taken when cleaning the media, so as not to compromise biological activity. Bacteria may be killed by washing filter media in untreated tapwater, excessively hot or cold water, or water of a different chemistry to that in the aquarium. 

Chemical filtration

Chemical filtration utilises the behaviour of certain materials to either adsorb (hold on to) or swap chemicals (by trading one toxic ion for another, less toxic one, for example) to remove specific wastes from aquaria. 
 
Chemical filter media usually takes the form of resins or pouches that are added to a filter to target a specific problem—zeolite, for example, will remove ammonia, while other resins may target nitrates or phosphates. 
 
Chemical filters are exhaustible, which is to say that they can only swap or hold on to a set amount of their target chemical before they stop working. This means that in order to function, chemical filter media needs to be routinely replaced. 
 
Carbon is a good example of a chemical filter media that needs regular replacement, typically having an active lifespan of around six weeks in aquaria (pending just how much waste it’s removing). While it is useful, carbon is not essential to the function of most commercial filters. 

Which filter media do I need? 

When it comes to choosing the right filter for your tank, in an ideal world, a good aquarium filter will include all three elements of filtration.
 
In order to keep a tank safe for fish and invertebrates, biological filtration is the most important of the three filtration types. Without it, ammonia and nitrite will not be controlled, and will rapidly reach toxic levels. 
 
Mechanical filtration is the next most important. In trapping wastes, it helps to protect the biological media from being smothered, and as long as the mechanical filter media is cleaned regularly, it will drastically decrease the burden on biological filter bacteria. 
 
Chemical filtration is arguably the least important of the three, and is usually only used for a purpose. If there’s a sudden spike of ammonia, or nitrates are excessively high, chemical filter media can be turned to as a short-term fix, but it is unlikely that a chemical filter will be the sole filtration for an aquarium any time soon.
 
A well designed canister filter will have the filter media arranged in the following order: mechanical, biological, chemical. This usually provides the optimal filtration for an aquarium.

Related article: Buyer's guide to external filters

Who needs a fish tank filter? 

Anywhere that fish and/or invertebrates are confined in a small body of water (such as a tank or pond) some form of nutrient export becomes essential. If this nutrient export is not immediate and continuous, then a filter is required.
 
Without a filter, living creatures will be exposed to increasing levels of harmful pollutants, chiefly ammonia. This ammonia will directly poison organisms, burning tissue and lowering immune systems. Eventually, diseases will break out in livestock, or it will be poisoned outright and die. 

Are there any aquarium tanks that don't need filters? 

There may be some tanks that can sustain livestock in in a less orthodox way than by passing water through an obvious filter system. In these tanks, there may be different biological processes occurring, or biological media might take a different form to the type of media used in a conventional filter system. 
 
Such examples include certain planted aquascapes, certain reef aquaria, and some extremely low-pH aquaria. 

Planted aquascapes

Plants utilise ammonia, nitrite and nitrate as a food source. In a heavily planted, high energy (with intense lighting, carbon dioxide supplementation and plant feeding to encourage heavy photosynthesis) aquascape with minimal fish waste (specifically, in a tank with a low stocking density) it is possible for the plants to consume all of the ammonia created by the fish. In such a tank, biological filtration may become redundant. 
 
This does rely on an incredibly low fish to plant ratio, and the plants need to be healthy, established, and growing for it to occur. 
 
Planted aquascapes may still need other types of filtration, however. Because leaves are constantly shed, and fish in such tanks still produce solid wastes, mechanical filtration is desirable to keep the water clean. The flow from a filter is also desirable for water circulation, transporting nutrients and dissolved gases around the tank.
 
One approach to creating an aquascape without a filter is to use the Walstad method, devised by Diana Walstad and pioneered in her book Ecology of the Planted Aquarium
 
By creating a balanced ecosystem between plants and fish, the Walstad method goes further than just not needing a filter. Carried out correctly, it also claims to remove the need to for water changes, although it is not entirely maintenance free—certain additional nutrients and minerals will be needed to be added to keep the ecosystem functioning.

Reef aquaria

While most marine reef aquaria are associated with vast and complex filtration systems, in the modern set up, the biological filtration is carried out by live rock, or one of its synthetic equivalents.
 
Live rock as found on coral reefs is the old aragonite skeletons of dead corals. In the same way as biological filer media is highly porous and has a high surface area to volume ratio, so too does live rock. 
 
Added in sufficient quantity, live rock will culture enough beneficial bacteria in its pores to convert all of the ammonia created by marine livestock, rendering other biological filter media redundant. 
 
Furthermore, while biological filter media can only go so far as to convert ammonia through nitrite and into nitrate, live rock also has the anaerobic (oxygen deficient) conditions for yet more bacteria to convert nitrates into harmless nitrogen gas. 
 
Some aquarists have managed to run lightly stocked marine aquaria entirely with live rock and a protein skimmer—what’s known as the Berlin method. A protein skimmer functions by removing amino acids directly from the water, before these acids have had a chance to break down into ammonia. In turn, this reduces the burden on the live rock. 
 
Some aquarists have gone even further and run reef tanks with nothing more than live rock and flow pumps, and in the right hands these can be incredibly successful. However, such tanks require a comprehensive understand of water chemistry and are not advised for the beginner.

Low-pH tanks

Most biological filtration is concerned with converting highly toxic ammonia into less toxic nitrite, and then again into much less toxic nitrate. 
 
However, there’s an interplay between ammonia and the pH value of water. Ammonia exists in two forms: highly toxic free ammonia (NH3) and barely toxic ammonium (NH4). When we test for ammonia using a test kit, we’re testing for what’s called total ammonia nitrogen (TAN), which is the sum of both ammonia and ammonium combined. To calculate the exact level of harmful free ammonia, we need to also know the temperature and the pH of the water. 

As water becomes warmer and more alkaline, the proportion of free ammonia drastically increases. 
 
For example, if we tested our TAN using a test kit and had a reading 0.1mg/l at 30°C, our free ammonia would be at the borderline safe level of 0.02mg/l. 
 
However, if we tested our TAN and had a reading of a massive 50mg/l at 20°C, our free ammonia would still be at the borderline safe level of 0.02mg/l. Temperature and pH have a huge impact on how much ammonia is free, and how much is not. 
 
In very acidic blackwater tanks with a pH of below 5.0 and a negligible KH (carbonate hardness) level, such as those blackwater systems for Boraras or cardinal tetra, some aquarists have found that tanks simply do not cycle in the way that tanks at a higher pH and hardness do. This may be in part down to the fact that biological filter bacteria are impaired at low pH values, but also due to the absence of carbonate hardness (KH is an essential part of the biological filtering process), as well as the fact that high TAN levels also inhibit bacteria development. 
 
Such acidic tanks present new problems, though. For one, not many fish can deal with such extremes, but also activities like water changing need to be watched closely. If an aquarist with a low-pH tank were to accidentally carry out a partial water change while TAN levels were high, and the replacement water was relatively alkaline, this would lead to the sudden release of harmful free ammonia, poisoning the inhabitants. 
 
There’s also some research in to nitrification in a Biofilm at Low pH that suggests that some nitrifying bacteria can indeed survive at low pH levels in some conditions. With this in mind, such acidic tanks should be closely tested, and if nitrification is occurring (this will show up as positive tests for nitrite) then a filter will need to be used to convert the ammonia and nitrite into nitrate through the usual means. 
 
There are few fish that require such extreme low pH conditions to thrive (some may only require it for spawning purposes) and many species can be kept at a more sensible 6.0-6.5pH, in which biological filtration can function normally. 

So, do I need a filter? 

Yes. There are very few instances where an aquarium filter will not be required, and all of these instances live at the extreme ends of the hobby. 
 
In these cases, while we might not be looking at a traditional filter per se, what we are looking at is a form of nutrient transport that replicates what a filter would do—detoxifying or removing ammonia and making water safe for a tank’s inhabitants. 
 
If you’re new to the hobby, or if you’re keeping a conventional aquarium within a normal pH and hardness range (which you almost certainly are) then a filter is essential in acting as life support for your fish, and that filter should at the very least be a biological filter. 
 
Related article: Learn more about cycling an aquarium filter