Filtration Basics

Filtration is the best way to maintain water quality in the aquarium – but many people starting out in the hobby are never told why, or get bamboozled by people talking about ‘the cycle’ or promoting their particular way of doing things. In its simplest form a filter is somewhere to grow beneficial bacteria, this could be a simple sponge filter through to a giant sump filter – they all do the same job different ways. Filter media is where the work is done, this is usually split into three types;

Mechanical: Inert media designed to physically pull debris out of the water column, usually sponge or filter wool.
Chemical: Chemically enhanced to remove or absorb impurities, such as activated carbon. Optional.
Biological: The most important element – this is what drives the cycle. This is inert media designed to provide the most efficient home to the bacteria that consume the harmful elements and break down toxins,

Hopefully this image explains a little what ‘the cycle’ is, and why it is important. The next question is usually ‘how do I do it’ so lets look a a few of the popular methods, and some misconceptions.

Quick start: This is usually broken down into two methods. Purchasing some kind of bacteria and adding it to a new system to seed  the filter, or being given some established filter media to add to your filter to achieve the same result. Pro’s and cons to both methods.

Fishless cycle: This is using pure ammonia to culture the bacteria within your filter before you add fish. You add ammonia (or old school style add some food to break down into ammonia – or people even used to pee in the tank!!) until such time as your filtration is clearing it and only showing nitrate in the tests. You need to do regular water tests looking for the ammonia to register, see that turn to nitrite, then finally nitrate.

Fish-in cycle: This is using a ‘hardy’ fish to produce the ammonia the filter needs to start the cycling process, which can tolerate the levels of ammonia and nitrite it has to live with. There are long running arguments about using this method, and whether the fact a fish can tolerate bad water parameters is good enough justification.

When you have ‘cycled’ your aquarium the end product is the production of nitrate – which is generally controlled by water changes and assisted by live plants if you have them, which will use the nitrates as a source of nutrition.


“Don’t change too much water – you will crash your cycle”
The bacteria lives in the filter, on surfaces and in the substrate – not in the water. I hear regularly from people who have had problems, and it turns out they have changed their filter. “But I used my old tank water” they will say – but unfortunately the water does not contain any useful quantity of the bacteria that keeps an aquarium healthy. They should have used their old filter media.

“Always wash your filter”
While it is a good idea to keep your filter running smoothly – often this advice is interpreted as an instruction to rinse out your filter media under a tap. Done in old tank water or some other dechlorinated source this is fine, but chlorine will kill filter media.

“You need to wait a couple of days before you add fish to a new tank”

This myth populated by some of the big chain stores is nonsense. Waiting a few days before adding fish to a new aquarium does ABSOLUTELY NOTHING!