This might sound like a really dumb question but I'm considering purchasing a Fluval external canister filter for my tank. I have an Aquaclear 150 and 200 on my 20 gallon goldfish tank and it seems like every week they are plugged up. I read that a canister filter is lot less maintenance but I have no clue how they really work. Do they mount on your aquarium? Do you use it with an air pump? The only pictures I could find were just pictures of the filters out of the box but not hooked up to a tank itself. Is it a wise investment or should I just let things be?

They're a real pain to start up. You get a siphon going and then screw it into the bottom of the cannister. The cannister sits below (or at a height lower than) your tanks. The siphon fills up the cannister from the bottom up. It passes through various mediums (i've go a scratchy sponge thing, cermaic noodle bits, a sponge from an old filter and some 'bio ball supplements') which mechanically and biologically filter it. Some people also put carbon in the cannister. Once the cannister is filled to the top, the pump at the top of the cannister pumps the water back up and into the tank.

I've got one and am not overly impressed with it. It seems to filter well, but it's a pain to start up. I much prefer my internal cannisters and my trickle filter. :)

i have a fluval 304 and it's a PITA to prime. i would say get yourself a rena filtar xp1, i have an xp2 and it's been kickass for me.

or if you have extra cash go with a smaller eheim classic.

In the case of a fluval cannister.

The cannister is a sepperate box with the media and a pump.
There is an intake hose and an output hose.
The water gravity feeds into the box and runs through the various media.
On the other side of all the media is the return pump and line.

As far as store bought filters go a sump is the best biological filter.
Ehiems are the rolls royces, I have used fluvals with out problems.