Filters work by: Chemical adsorption, mechanical removal of particulate matter and by providing bacteria a place to hang on creating a "bilogical filter" that breaks down harmful chemicals like ammonia into less harmuful or "inocuous" chemicals like nitrates.

Chemical adsorption ( notice adsorption, not absortion ) is achieved through materials like charbon, earths like zeolite or resins that bind chemicals to their surfaces, some, like charbon, once their surface of contact is depleted are no longer useful and have to be replaced with relative frequency, others like earths and resins can be "recharged" extending the life of the product, obviously with time they will also saturate and will become useless.

Mechanical filtration is achieved by forcing a flow of water through a material to trap the suspended and particulated matter, the material can be gravel, synthetic wool or sponges, they also need cleaning to remove the trapped particles and, when necesssary, replaced.

Biological filtration is done by bacteria, called nitrifying bacteria, the bacteria break down ( feed on )ammonia, oxydize it and turn it to less toxic nitrites, which are later oxydized and turned into "inocuos" nitrate, "inocuous" doesn´t mean non toxic, it means that the levels of tolerance of animals and invertebrates to them is much higher. Nitrifying bacteria has to be cultivated and grown in the aquarium, it comes with the water you pour into the tank or it can be seeded adding filter media from an established aquarium or purchased as a freeze dried or liquified product, in order to grow it needs three things: a surface to attach to, oxygen because it is aerobic ( it breathes oxygen ) and a food source either provided by you or by the animals in the aquarium.

An aquarium is only an artificial ecosystem that mimics incompletely what happens in nature, I mean incompletely because in order to mimic perfectly what happens in nature you need more filtration equipment to continue the cycle until all nitrogenous compounds are turned into nitrogen and oxygen gas. Sulfur smell means that there´s an anaerobic process taking place and it´s definately not a good indication, it indicates a reductive environment and reductive environments are not suited for oxygen breathing life forms.

Thanks! Great answer!

It's okay not to have the protozoans and pond bacteria in your tank. More often than not, that leads to disease. What you want to do is start a cycle, meaning you want to grow the nitrifying bacteria so that they will eat the ammonia for you. When they eat ammonia, it gets turned into two compounds, nitrites and nitrates. When your ammonia and nitrites are 0 and your nitrates are at a low level, that is considered a full cycle and an established tank.

I'm not the world's best expert on cycling, so I'll let someone else who knows more do the thorough explaining.

To add to what Raul has said, if using carbon, use activated carbon and NOT charcoal. It is easy to tell the difference as charcoal has a glossy shine to it and activated carbon is dull. Charcoal will work, but not for long as there are many fewer adsorption sites in charcoal than there is in activated carbon. Typically, carbon will work 4 to 5 times longer than charcoal and is not much more in cost. Also, activated carbon can be recharged and reused, but it is not worth the trouble.

Also, sounds like you may have a Whisper filter or something similar. To keep the tank from continually cycling when you change the cartridge, place a small amount of filter floss in the filter after the cartridge (with respect to the flow of water). When changing the cartridge, always leave the floss in so you have bacteria in the filter.

Joel

The difference between charcoal ( charbon ) and "activated" charcoal or charbon is this: "regular" charcoal or charbon is just that, charbon, a carbonized chunk of animal or vegetable matter, like HS said, it´s easily recognizable by it´s familiar form and glossy surface look, it has all the attributes of charcoal ( chemical adsorption ) but with one big disadvantage = poor surface area, so it´s lifespan is limited and it´s attributes dissapear really fast.

"Activated" charcoal is made from common and ordinary charcoal but it´s processed, charcoal chunks are ground to dust, then that dust is compressed, cooked at high temperatures and pelletized in an extruder machine, to "activate" then it´s washed in a phosphoric acid bath that "opens" the grain to make it porous and then rinsed in running water to remove the acid, later it´s dried with air and finally the pellets are crushed to a certain size; the final result is a product that keeps the chemical adsorption properties of charcoal but with a huge contact surface, the comparison would be that a pound of activated charcoal has 30 time more contact surface than it´s equivalent in "common" charcoal. Like HS said, activated charcoal can be "recharged", it´s true to a certain point but the "recharging" is so poor that it´s not woth the effort, charcoal can´t be as easily recharged as earths ( zeolite) because the way it adsorbs is different, adsorbtion in charcoal takes place by a "strong" electrical bind between the substance to be adsorpted and the surface of the charcoal while earth do the same but with a "weak" electrical bind, terefore it´s easier to break the bind in earths.