Agree with Andy. When you have a new setup you are not familiar with yet, an initial clean and then a pattern of one cleaning per month is a good inital trial frequency, all the while watching to make sure the water flow does not slow significantly, prior to the one month mark. The initial cleaning should be quite gental since you may not fully know how healthy or mature the fitler is. A bucket of tank water (the water that was just removed via a gravel-clean-siphon-out process) is used (definately not tap water!) When you open the filter, be careful to observe everything about it (take notes if necessary.) You want to be able to put it back together in the same way and you don't want to break anything. Media that are sponge-like (sponges or felt baglike structures etc.) are gently squeezed in the bucket of tank water such that you get the loosse debris out but don't roughly wring everything out of the sponge. The beneficial bacteria will cling tightly to the sponge itself or other material, so there's also no need to worry excessively about this process. If you have "loose media" such as ceramic gravel or ceramic rings, these can be dunked and swished to clear them of debris.
Ideally, with each filter clean, the impeller assembly should be removed and it and the well it sits in should be cleaned with small brushes. If you don't yet have the right brushes, don't fret, its still better to be cleaning your fitler even though you were unable to fully clean your impeller. Likewise, if you have silicon seals on your filter anywhere, its good to use a "Vasoline"-like silicon lubricant to refresh the seal material so that it won't oxidize. Often the "fine mechanical" layer of filtration (often a floss pad) will need to be replaced, although sometimes it can also be rinsed out and re-used. It should be obvious that there is a large sponge or gravel layer that serves as the "biomedia" where most of the bacteria are housed that will be retained and not replaced.
Hope this helps a little,
~~waterdrop~~
