You don't change it- the filter is where all the good bacteria live, the ones that keep your fish from getting poisoned. The only type of filtration you need to change very often is carbon filtration (it'll be a black pad, a bit like the stuff you get in cooker hoods), and you don't need to use that at all. Plain sponges and bio-balls or other ceramic media are all you need. The sponges should be changed when they lose their elasticity, maybe once a year or 18 months, and then you replace it gradually by cutting one in half and replacing it, then the next week the same again etc.
To clean the filter you should give the sponge (and ceramic stuff if you have it) a rinse in water taken from the tank (NEVER EVER tap water, the chlorine will kill all the good bacteria) when you do a water change, once a week is normal but more often if your tank is heavily stocked. What sort of fish do you have?
Thanks, that is really useful. We have platy's which seem to be breeding a lot, minnow, tetra's and guppy's oh and a catfish. There are also rogue snails. The gravel seems to get dirty quickly which I hoover but was not sure how I tell when to change filter. Will do as you suggested. Any ideas on how to make the air bubble pump outside the tank quiet?
You can suspend you air pump from a bit of string to keep it quiet but make sure the air pump is above the level of the water otherwise the air line can siphon the water from the tank if the pump fails or you have a power cut, you can also fit a non return valve on the air line if it is not possible to put the air pump up high.
bureni, good tip. We came home one day to discover the tubes had detatched and we had lost 160 gallons of water and had fish cowering at bottom. God knows where the water went under the floor as had no sign of dampness anywhere. Having just had power cut tonight wouldn't want a repeat so will try suspending it.