Filters are an essential piece of equipment for any aquarium including smaller or nano aquariums. We wrote an article here that describes the purpose and benefits of having a filter installed. However, there are many different types of filters on the market and it can be difficult to decide on which is best for your aquarium. Knowing the advantages and disadvantages of each type is important given that you don’t want to waste money on purchasing a filter that is not appropriate for your tank.
Here is a list of the most commonly used filters for smaller aquariums.
- Internal filters
- Hang-on back
- Sponge
- Cannister
To find some actual usage numbers, we performed a poll in various fish-keeping and aquarium groups on social media to see which types of filters were the most popular. The poll query was:
“Which type of filter would you choose for a 20 gallon planted tank with a pair of centerpiece fish and a handful of cleaner crew fish.”
These are the results:
Read on to learn more about each filter type so that you can choose the most appropriate and practical filter for your aquarium.
The Type of Filter depends on your Aquarium Goals
Any filter will do its job just fine, but how do you know which filter will fit your particular needs? Narrowing down the options depends on what your tank is used for and what you are trying to achieve.
If you are aiming to produce a lush and heavily planted aquascape then it makes sense to choose a smaller filter, or opt for an external filter altogether. If your tank is to be used as a hospital or quarantine tank then looks probably don’t matter and a simple and inexpensive sponge filter will be just fine. If your tank is sitting on a desk and you really don’t have space underneath to store an external cannister filter, then a hang-on-back filter may be the filter for you. Have you considered ease of use and how much maintenance you need to perform? Is there a possibility in the future where you may want to include shrimp?
Ask yourself these questions as they are important to deciding which filter is best for your needs.
What to Look for in a Filter for a Small Aquarium
In addition to your goals for your nano aquarium, you should consider the points below when selecting your filter. These are key things you should look for in a filter.
Will it be Inside or Outside?
Knowing where your filter will be placed will narrow down your options. Where will the filter be situated? Inside or outside your tank? As mentioned above, this is a key factor when determining what filter you purchase. This is because certain filters are specifically designed to be inside or outside the aquarium.
An aquarium that is sitting on top of a cabinet or stand should be large enough to house a cannister filter. Only the inlet and outlets of a cannister filter will be situated inside the aquarium. The hang-on-back filter is situated where the name suggests – on the rear wall. A small space at the rear of the tank will be needed for the body of the filter to rest while the inlet pipe is the only part that is inside the tank. Unlike the cannister filter, water is allowed to cascade back into the tank via an overflow which creates surface agitation.
- Inside aquarium – internal, sponge, and under gravel.
- Outside aquarium – hang-on-back and cannister.
Flow and Surface Agitation
Depending on the fish you currently have or plan to keep in your nano aquarium, the flow rate of the filter will affect your fish and plants. For example, a cannister filter with a high flow rate will likely stress out fish that prefer slow-moving water (e.g. Bettas). In this case, a sponge or hang-on-back filters are not capable of producing high flow rates and would be more suitable.
High flow rates are typically associated with cannister filters. Flow rates are measured in Gallon Per Hour (GPH) and different sized cannister filters will have different GPH ratings. Look for the GPH rating on the box packaging. If you end up choosing a cannister filter then it is best to pick a GPH rating that is at least 2 times the volume of your tank. This means that it will be able to filter twice the tank’s volume in one hour.
Most cannister filters come with taps on the outlet pipes so that the flow rate of water entering the aquarium can be adjusted. If your cannister filter doesn’t come with this feature then it’s not the end of the world as taps can be fitted with a bit of DIY.
Generally, most filters will eject water back into the tank to create agitation at the surface. This process helps to promote carbon dioxide to leave the water and oxygen to enter the water by diffusion. Alternatively, some filters like sponge may create air bubbles that perform the same function.
Cost
Depending on your budget, you may be limited to a few types of aquarium filters. Below are example prices (at the time of writing) on Amazon (excluding shipping):
| Internal | Fluval Nano Aquarium Filter – $38 Aqueon Quietflow Internal Power Filter – $64 Fluval U1 Underwater Filter – $67 |
| Hang-on-back | Aquaclear 100 – $23 Aqueon Quiet Flow 20 – $39 Seachem Tidal 35 – $58 |
| Sponge | Pisces Aquatics E-Jet Air Sponge Filter – $12 Aqua One Filter Air 30 – $19 |
| Cannister | Fluval 107 Series – $110 Eheim Classic 150 – $232 |
Types of Filtration Offered
Mechanical and biological filtration are the two most important types of filtration. Most filters will offer mechanical filtration (e.g. filter pads and sponges) however more is better. Because the more there is, the greater surface area for beneficial bacteria to grow and provide a means of biological filtration. Cannister and hang-on-back filters will usually come supplied with a couple of types of filter pads. What you ideally need is a combination of a coarse filter pad and a finer filter pad. The coarser pad will filter out and hold all of the larger waste products in your tank, for example, excess food, parts of plants. The finer filter pad will remove the smaller particles and help to ‘polish’ your water.
The third type of filtration is chemical where dissolved particulates are removed from the water column. This is where you can insert activated carbon or a product such as Seachem Purigen to polish your water. In most cases, chemical filtration is not needed on a regular basis and is often used as a measure to remove medications, tannins, and other dissolved impurities from the water. Once the product has absorbed its capacity for impurities then it will need to be replaced.
Cannister and hang-on-back filters have the capacity to keep a variety of filtration media. And best of all, they are highly customizable to your filtration needs. If you want to remove a lot of particulate matter then you can switch out a finer filter pad for a more coarser pad. Or, if you have just placed some driftwood in your tank and your water is starting to turn a slight brown color, then you can replace a pad with some activated carbon or Seachem Purigen.
Sponge and internal filters have no way of adding a means of chemical filtration.
A Rule of Thumb on Capacity
The purpose of a filter is to improve the quality of the water in your aquarium. Wastes such as excess food, decaying plants, and fish excrement build up over time. So it’s important to get a filter that is rated to at least the volume of your tank, if not more. If your tank is 20 gallons (90L) then you should be sourcing a filter that is rated for at least 20 gallons. For example, the Ehiem 150 Classic cannister filter is rated between 11 and 32 gallons which would make it a suitable option for a 20-gallon tank.
If the rated capacity of a filter is too small for the volume of your tank, then waste will build up over time and lead to issues including excessive algae growth or fish deaths.
Types of Filters
The Most Popular: Hang-On-Back
Hang-on-back filters sit on the top of the aquarium, with the intake tube inside the water and the pump motor and filter material outside. They are one of the more commonly used filters for their ease of installation and use, and high level of customizing the type of filter material used. Most models will come with a flow controller so you can decrease or increase the rate of outgoing water flow, which can be important depending on the type of fish you have.
The customizability of the filter material makes the hang-on-back filter allows you have all three types of filtration. You can insert a coarse and fine filter pad, followed by a bag of Seachem Purigen to really ensure the best water quality possible. This is a clear advantage over the sponge filter where you only have the choice of either a coarse of fine sponge.
The box where the filter material is stored is easy to access, inspect and remove. When removing it for maintenance, you may find some water drip outside the tank, however this happens with anything you take out of a tank. Our recommendation is the Seachem Tidal 35, where the filter cover can also be used as a tray to catch watch when removing the box.
Popular hang-on-back filters such as the AquaClear have the motor in the filter box outside the tank. The potential issue with this is that the filter pads may dry out when a power outage occurs. The filter box becomes dry during a power outage as water is no longer being pumped through. As a result, the motor will likely burn itself out. In terms of reliability, motor burnouts or failures are the most common issue for hang-on-back filters. The Seachem Tidal range has its motors inside the aquarium and is therefore in water all the time. When power returns, they will self-prime and recommence filtering.
If you have shrimp or fish fry, you can easily attach some sponge around the water intake tube.
Our pick: Seachem Tidal 35
In Case of Emergency: Internal
Internal filters are easy to install and use and cheap, however not versatile compared to other filtering options for a small aquarium. The pump is often located at the base of the filter. Water is drawn in, passes through a small number of filter pads, and ejected at the top and back into the aquarium. Filter pads are typically used however their small size doesn’t allow for a whole lot of pads to be used or different sizes. Chemical filtration is therefore not an option with internal filters and you will need to come up with another solution if you needed this.
Like sponge filters, internal filters are usually kept in the back corner and hidden out of sight using plants, rocks, and driftwood.
Maintenance will be required on a regular basis to clear out debris from the filter pads. This is best done by washing them out in old tank water during a water change so the beneficial bacteria are not harmed.
Despite their downsides, internal filters are to easy to place around the tank and are not destined to be fixed in one particular location. This makes them great if you need the extra filtration and water flow around your tank. This is reflected in our poll where internal filters were not considered the best filter to permanently have in your nano aquarium.
Our pick: Fluval U1 Underwater Filter
Cheap and Shrimp Friendly: Sponge
The humble sponge filter is probably the most common filter used by beginners. They are incredibly cheap, easy to use, and will last a long time since they don’t have a lot of moving parts. For these reasons, you will commonly see sponge filters being used in fish stock tanks in your local aquarium store. They provide lots of mechanical and biological filtration, as well as surface agitation.
Sponge filters comprise of three main parts:
- A sponge that sits inside the tank.
- An air pump (can be battery operated) sits outside the tank.
- Air tubing that connects the air pump to the sponge filter.
So how does it work? The air pump pushes air through the tubing which goes through the cavity inside the sponge filter. The air bubbles that are produced from the air pump draws in water from the surrounding sponge filter and therefore creating a means of mechanical filtration as debris is collected. The sponge also provides an area for beneficial bacteria to grow on.
Sponge filters are a popular choice for shrimp tanks. This is because the drawing in of water is slow compared to other filters, but more importantly, shrimp are not likely to get sucked up into the filter. Instead, you will commonly see shrimp happily grazing algae on sponge filters. Sponge filters are also used in fry tanks for the same reason.
Unlike most hang-on-back filters which will dry out in a power outage, the beneficial bacteria living on the sponge won’t be affected since the sponge sits inside the tank.
Sponge filters are reliable and very cost-effective, however, they do take up a lot of space in a small aquarium. They are not the prettiest products either and are usually hidden in the back of the tank behind plants, driftwood, or ornaments where they cannot be seen. Since sponge filters produce air bubbles, they can be a bit noisy. However, this can be remedied if you insert a small air stone into the tubing.
Another downside to sponge filters is that you are unable to have multiple stages of mechanical filtration. Let’s say you have a coarse sponge installed. In this case, finer particles in the water will take longer to be collected or not at all. Alternatively, if you opt for a finer sponge then larger particles may not be collected, and the sponge will clog up faster. Cannister and hang-on-back filter do not suffer from this disadvantage, as you can use a combination of coarse and fine filter pads to filter out debris of varying sizes.
Do sponge filters need cleaning? Yes, they do. You will be able to tell when cleaning is needed when you can visibly see debris (essentially brown gunk on the sponge) on the sponge. Or, the bubbles coming out the top have decreased in number and size which means that the sponge is starting to be clogged and needs cleaning. Cleaning your sponge filter is best done during a water change:
- Remove the air tubing from the sponge. Leave the tubing (and air stone if you have one attached) in the tank.
- Take the sponge and squeeze it out in a bucket containing old tank water that you removed from the water change (never use tap water on your sponge as it will kill the beneficial bacteria).
- Repeat this until the water being expelled from the sponge is clear.
- And that’s it! Reconnect the air tubing to the top of the sponge and place it back into the tank.
Sponge filters do not have a lot of moving parts so they will usually last a very long time.
Our pick: Pisces Aquatics E-Jet Air Sponge Filter
The Premium: Cannister
In terms of filtration capacity and performance, cannister filters are the best filters you can buy. The space inside the filter allows you to store a lot of filter media for mechanical and biological filtration whilst providing the option of chemical filtration in the future. Simply replace a filter pad with a small bag of activated charcoal or Seachem Purigen.
Cannister filters are known for their big and bulky size. However, they are situated on the floor or inside the cabinet below the aquarium and out of sight. The only parts that are inside the aquarium are the water intake tube and the outflow tube. If you don’t like the color of the standard intake and outflow pipes (Fluval pipes are dark olive green), then you easily find clear glass or stainless steel pipes online or at your local fish shop.
Maintenance can be a bit tricky as you will need to shut off the water and lift the lid off with water being spilled everywhere.
For a aquarium that is less than 10 gallons, then the cannister filter is probably a bit of overkill.
Conclusions
If you have a small aquarium of 20 gallons or less then look no further than the hang-on-back filter. They offer more filtration versatility, are highly customizable, and are easy to use and maintain. Their design also allows them to stay hidden for the most part, at least compared to the internal and sponge filters which take up a lot of valuable space. Although slightly more expensive, the Seachem Tidal 35 is our recommendation in this category due to its features and well thought out design.