Re: could someone please explain a canister filter
Its like an overflow, box sort of. Instead of the box on the back of the tank, you will usually have tubes going down to the canister filter. Also you have alot more flexiblity in what you want to put in your filter.
The filter pulls water from the tank, usually pushes the water across your media and puts it back into the tank. You usually have baskets that you put your media into. If you don't want carbon you don't have to, you can instead put in lava rock.
Its very useful and very nice, depends on the brand, (usually quiet) and goes in the cabinet underneath your tank.
Number of good brands out there I recomend the Eheim myself. Hope that helps.