There are so many types of filtration methods out there. You have hang on back filters, sponge filters, internal filters and sumps. This post will focus on three types of filtration, HOB, sponge and internal. First off, here is the basics of filtration. You have a water pump that will force water through the mechanical media (sponge etc) this is used for pulling out particals and will house the majority of your beneficial bacteria and chemical media (I.e activated carbon). One thing to note is that chemical media like activated carbon, is not essential unless removing medication or other impurities from the water, despite what the manufacturers say, then after passing through the media it is sent out the output back into the tank. The nitrogen cycle is taking place all the time (click here for info about the cycle) and most of the benafical bacteria will be housed in your filter due to the pours in sponges and other media, but beneficial bacteria can be found on every surface of your tank. A key factor for filtration is the litres per hour. You never want to under filter a tank, you can't do much harm over filtering (unless you use a massive pond filter for a 40 litre tank). Remeber that bigger the tank bigger the filter you will need. Aim to get a filter that can filter your tank at least 4x per hour. So for example if you have a 100 litre tank, you will want to get a filter that can do no less then 400 litres per hour. HOB Filter (Hang on Back)HOB filters are commonly used for tanks that are rimless or have no lid. They normally have an intake pipe that will sit inside the tank, this will suck up water and pass it through the filtration media. But the output is a water fall style. This is good for two things, oxygenation and surface agitation. One problem is that HOB filters don't provide much of a current in the water. Because of the way HOB output is designed (water fall style) it allows the water to become oxygenated, but because there is very little current being produced you may find that there are dead spots in your tank where oxygen is not present or very little amount can be found. With the force of the water falling, it disrupts the surface causing small ripels, this helps with gass exchange and heat exchange. Internal filtersInternal filters are pretty standard when it comes to home aquarium. The principal is the same as any filter but internal filters are submerged in the tank. This is some times seen negatively as it takes up room and is not very appealing. But they have huge benefits. First of all they can provide strong water circulation. Also they can create good surface disruption, when the outlet is close to the surface of the water. The force of the water being pushed out causes the water surface to be disrupted. Internal filtersare not always the best, as snails, shrimp and fry can get sucked up into the filter, but there are easy ways to fix this. Sponge filtersThe way that sponge filters work is by using water displacement, by pushing air into a cylinder it forces water to be moved out the path of least resistance (outlet tube), but if you have ever pushed a empty cup under water, then you will have seen air rise and water fill the cup. With a sponge filter, it works by "emptying" the small space inside the filter, but because of this presure and the fact that air is lighter then water, the air is going to escape through the outlet pipe, which will also force water to go up with it, now the filter needs to fill up, so it will suck water through the sponge. Sponge filters also provide good feeding ground for fish and shrimp due to the sponge being exposed, it allows them to pick at the debris stuck to the filtering. Due to the sponge being exposed when you go to lift the filter out of the tank, you will often find that a lot of the debris will fall off, which is a pain to fish out. But I personally think that this is a small draw back. A sponge filter allows you oxygenate the water, provide surface agitation and create a nice feeding ground, so all in all they are quite good. So that is the basics of filtration. If you have any questions feel free to ask, better to ask then not to know.