"only been about 4 months"
Normal filter maintenance varies all the way from possible weekly cleans needed for very small filters in heavy fishload situations to possibly going 6 months or a year before mainenance on a large sump filter, filtering a lightly loaded tank, so it can vary a lot.
However, a good starting point for all beginners who have followed normal guidelines and sized their filters appropriately for their fish load and tank size would be 1 month (4 weeks.) When cleaning the filter its just going to be a very subjective thing deciding whether its telling you to adjust your maintenance interval upwards or downwards. If its not obviously clogged and doesn't seem excessively overloaded with debris to you then staying at one month is probably fine. The one-month guideline is probably most fitting for larger external cannister filters and larger HOB filters. Smaller internals might need checking at two weeks until you've determined that they're up to the one-month interval.
When cleaning filters, the biomedia is always cleaned in tank water. Tap water with chlorine or chloramines would risk killing off too much of the beneficial bacteria, especially for the beginner with fledgling bacterial colonies. Sponges are squeezed out in tap water. If the filter is "young" the squeeze should be more gentle, if its "mature" (say, after year, which is generally what we mean by mature) then the squeeze-out can be strong. Ceramic rings, ceramic gravels and bioballs are generally dunked and swished in tank water. The dunking process is made easier if the filter provides mesh media bags or easiest of all if it has media trays.
Many filters include a final pad or filter floss for catching the very small debris particles before the filtered water returns to the aquarium. Sometimes this is called a "water polisher" (to make it sound even more effective than it is.) Its a judgement call whether and when to replace this pad or floss. Normally, in larger filters, the floss or pad does not represent a significant fraction of the biomedia and thus can be changed without worry of too much bacterial loss. Some people with heavy fish loads change this at every filter clean but others hardly ever change this component either. Polyfloss (which, by the way, can be purchased most cheaply as pillow stuffing (be sure to get chemical free type) at big box stores like Walmart in the U.S.) can often be added as the last thing in a filter, regardless of whether the filter manufacturer had this as a feature when the filter was sold.
Its a good idea to wipe or wash down the filter box itself and any plastic parts you can get to. This can be done under the tap - its only important that the biomedia itself not be exposed to too much tap water. Each manufacturer will have different recommendations for impeller care and beginners should be very cautious and gentle around this little propeller when they are first learning to clean. Each cleaning should include the re-lubing of all silicone O-Rings, which are the rubber-looking seals that keep water from leaking on many filters. Mostly its external cannisters that have these as an important aspect. Manufacturers will supply a clear jelly-like lubricant for this or you can use vasoline. You take the O-ring off and coat the entire thing between your fingers and then carefully return it and seat it properly. By the way, you should be keeping your biomedia submerged in tank water during all this and so its a good idea to always siphon your tank water into a bucket in the bathtub or something on filter cleaning days. Its not critical that biomedia be submerged every minute however and beginners shouldn't be paranoid about bacteria dying.
Among advanced aquarists, there are varying schools of thought about filter maintenance I believe. There are those with lots of large fish, huge sump filters and massive biomedia volumes who are quite happy going very long times between any maintenace of thier sumps. There are those that have massive algae filters with lights shining on them who have to have a different schedule to keep the algae at a certain thickness I believe, and other exotic stuff like that. In contrast there are hobbyists in the high tech planted track of the hobby who like to keep their filters very clean so that pockets of ammonia and nitrate in the filters themselves won't serve as a trigger for algae in the tank I believe. There are even hobbyists in the natural planted tank track of the hobby who lower their biofiltration to a minimum to keep it from competing with the plants for the Nitrogen nutrient available in the ammonia. All of that can be exciting stuff to learn about but I think are reasonable starting point for the beginner to the hobby is the 1 month filter cleaning guideline, for starters.
~~waterdrop~~ ("will someone please tell him to shut up and give it a rest!"
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