What is a filter? A filter can be fitted into any aquarium and is essential for keeping the water healthy and keeping your fish alive! It has a built-in pump which circulates and oxygenates the water, collecting all the waste into sponges or 'media' in a compartment inside. You change or clean the media when you do your monthly water change and maintenance. It's a bit like the way a tumble-dryer whizzes all the loose fluff into that little mesh bit that you have to empty now and again - the machine is doing all the hard work and you just have to empty what it's collected.
Which one to choose? There are hundreds of makes and models to choose from, but these here are our favourites. The Ciano range are physically very small (so not too ugly in your tank) but powerful and simple to use. They have optional extra cartridges that you can add or swap to solve particular problems, e.g. to treat algae or cloudy water. They are quiet, reliable and cheap - what's not to like!?
For larger tanks or where you have big greedy fish that produce a lot of waste (goldfish!) we would recommend the Eheim Pick Up range, which are like the Volvo of the aquarium tech world - no frills but very dependable and excellent value for money. The media is just a simple sponge - and unlike some flashy and over-priced rival brands, you aren't tied into buying expensive monthly consumables.