Practical Fishkeeping Diploma - Week 2, Day 5

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Here's the last lesson of week two of the Practical Fishkeeping Aquarist Diploma. Next week, we cover fish habitats and physiology.

Week 2, Day 5: Filter designs

Internal canisters

Internal canister filters are chambers of media with a built-in pump at the top or bottom. They remain submerged in the aquarium and need to be removed for maintenance and cleaning.

Pros: Discrete. Affordable. Easy to maintain. Low running costs. Not very time consuming to work with. Some designs offer versatile media types.  

Cons: Limited filter capacity. Difficult to hide. May dump waste back in tank while being removed for cleaning. Often restrictive of media that can be used. Require regular cleans, some designs can prove a hazard for fish that trap themselves behind the filter. Relatively limited flow control, if at all. Some models have restrictive choices of media. Multiple filters may be needed to run large aquaria.

External canisters

External canister filters are large, sealed chambers connected to a tank via a series of hoses and powered with either a built-in or inline pump. They can be disconnected from the plumbing for maintenance reasons. While small models exist, they are usually aimed at larger aquaria.

Pros: Large capacity for media. Wide choice of media options. May run for several weeks between cleans. Only the pipework and inlets/outlets are visible inside the tank. Water returning to the tank may be distributed forcefully through a nozzle (for high flow) or softly through the likes of a large spray bar (for reduced flow).

Cons: Expensive to buy. Running and replacement media costs can be high. May be unsuitable for small aquaria. Loose pipes or seals may cause catastrophic leaks. Heavy to carry for maintenance when full. Some models require time-consuming priming after each clean.

Air powered

Air driven filters may be box or sponge based, and sit inside the aquarium, connected to an outside airpump via a length of airline.

Pros: Incredibly cheap. Multiple filters can be run from one power source. Because they produce a slow flow, they are good to use with fry. Very easy to maintain. Box filters can be loaded with any media you choose, taking any role. One design, the Hamburg Matten Filter, even becomes coated in a nutritious biofilm for fish fry to eat.  

Cons: Limited flow rates. Often a small media capacity. Usually unsightly. Many box filters are transparent in design making them susceptible to light contamination. Require an airpump to function — not self-contained like internal or external canisters.


Undergravel filters are plastic plates with perforations that sit under aquarium gravel and draw water through via one or more uplifts. They are either driven by airpumps or powerheads and generally considered obsolete. This type of filter uses the aquarium gravel as a mechanical and biological substrate.

Pros: Very cheap to buy and run. Few working parts to break. Massive biological area for bacteria. Almost entirely hidden from view. Can be run ‘reverse flow’ with water pushed downwards through the uplifts and up through the gravel, nulling many of the cons listed below.

Cons: Requires incredibly high maintenance. Unsuitable for most plant types. Traps waste under the plate where it cannot be easily removed. Difficult to incorporate chemical filtration. Requires specific gravel depth and grain size. Hard to replace if damaged.

Fluidised beds

Fluidised bed filters are rarely used in freshwater aquaria, and either function biologically or chemically if special media is used. They usually sit outside of a tank, or as part of a sump system. Sand is the commonest substrate used in freshwater, and is kept in constant suspension by a flow of water from bottom to top.

Pros: Incredibly high biological surface area. ‘Self-cleaning’ as dead bacteria sloughs off of sand as part of the water flow and abrasion of sand.

Cons: Extremely limited mechanical filter action. Physically tall and hard to conceal. Requires specific flow rate to function. Easily compromised if flow is incorrect.

Trickle towers

Trickle towers (also known as wet and dry filters) process ammonia faster than submerged filters. A trickle tower utilises biological substrate stacked in such a way that water can trickle down over it, soaking it and bringing nutrients to the bacteria that live inside. Because the substrate is exposed directly to air, the bacteria are totally uninhibited for an oxygen supply.

Pros: Unrivalled biological activity. Hidden outside of tank. Easy to work with and clean.

Cons: Consumes lots of space. Requires sump system and hard plumbing to function. Creates large amounts of evaporation. Can be noisy.

Other specialist types of filters include socks, drum filters, biological wheels and bead filters. While these have applications that are often commercial, they rarely make an appearance to the home aquarist.

That’s it for week two. Next module: Fish habitats and physiology

Click here for next lesson: Aquatic habitats

How to gain your diploma: Once all the course modules and revision pages have all been posted online, we will open a link to a website that allows you to take your free online exam. If you pass the exam, you will digitally receive your very own Fishkeeping Diploma, to show that you have successfully completed the course, and which is yours to display on the wall near your aquarium, hang in your fish house — or keep somewhere safe where you can take it out and just look at it from time to time.

Note: The Fishkeeping Diploma is not a formal or accredited qualification and should not be confused with the type of diploma presented by colleges, universities and other educational establishments.

Further reading from our sponsors

Filtration for your aquarium

Filtration products for your aquarium

Universal dosing calculator